Add priliminary support for perforce SCM

This patch adds very simple support for the Perforce SCM:
    https://www.perforce.com/

Although perforce is proprietary software, it's somewhat prevalent in enterprise
companies. This patch looks to provide some basic bash_it functionality that
I've come to love for git. I base everything off of two perforce commands:
    $ p4 set
This command does not require a connection the perforce server, it simply tells
us if a directory is managed by the Perforce SCM or not. In addition the
command:
    $ p4 opened

is used to provide the list of pending changes in the client and the number of
opened files in the client. The `p4 opened` command requires a connection to the
perforce server, hence it's run under a `timeout` command. The "p4 opened"
processing into it's own bash file that now has to be sourced at the top-level
bash-it.sh. Since the processing in simple the newly added: _p4-opened-counts
function returns a number of things that are not currently used, but since I had
awk open and doing the processing, I've chosen to include them in the output
anyway.

Testing:
  - Tested with the powerline-multiline theme in a few perforce based
    workspaces/clients
  - Ran:
      ❯ shellcheck themes/p4helpers.theme.bash
    and fixed all the errors
  - Ran the test suite:
      ❯ test/run
      [...]
      182 tests, 0 failures, 1 skipped

themes/base.theme.bash

@@ -42,6 +42,12 @@ SCM_GIT_STAGED_CHAR="S:"
 SCM_GIT_STASH_CHAR_PREFIX="{"
 SCM_GIT_STASH_CHAR_SUFFIX="}"
 
+SCM_P4='p4'
+SCM_P4_CHAR='⌛'
+SCM_P4_CHANGES_CHAR='C:'
+SCM_P4_DEFAULT_CHAR='D:'
+SCM_P4_OPENED_CHAR='O:'
+
 SCM_HG='hg'
 SCM_HG_CHAR='☿'
 
@@ -71,6 +77,7 @@ function scm {
   if [[ "$SCM_CHECK" = false ]]; then SCM=$SCM_NONE
   elif [[ -f .git/HEAD ]]; then SCM=$SCM_GIT
   elif which git &> /dev/null && [[ -n "$(git rev-parse --is-inside-work-tree 2> /dev/null)" ]]; then SCM=$SCM_GIT
+  elif which p4 &> /dev/null && [[ -n "$(p4 set P4CLIENT 2> /dev/null)" ]]; then SCM=$SCM_P4
   elif [[ -d .hg ]]; then SCM=$SCM_HG
   elif which hg &> /dev/null && [[ -n "$(hg root 2> /dev/null)" ]]; then SCM=$SCM_HG
   elif [[ -d .svn ]]; then SCM=$SCM_SVN
@@ -81,6 +88,7 @@ function scm {
 function scm_prompt_char {
   if [[ -z $SCM ]]; then scm; fi
   if [[ $SCM == $SCM_GIT ]]; then SCM_CHAR=$SCM_GIT_CHAR
+  elif [[ $SCM == $SCM_P4 ]]; then SCM_CHAR=$SCM_P4_CHAR
   elif [[ $SCM == $SCM_HG ]]; then SCM_CHAR=$SCM_HG_CHAR
   elif [[ $SCM == $SCM_SVN ]]; then SCM_CHAR=$SCM_SVN_CHAR
   else SCM_CHAR=$SCM_NONE_CHAR
@@ -93,6 +101,7 @@ function scm_prompt_vars {
   SCM_DIRTY=0
   SCM_STATE=''
   [[ $SCM == $SCM_GIT ]] && git_prompt_vars && return
+  [[ $SCM == $SCM_P4 ]] && p4_prompt_vars && return
   [[ $SCM == $SCM_HG ]] && hg_prompt_vars && return
   [[ $SCM == $SCM_SVN ]] && svn_prompt_vars && return
 }
@@ -125,6 +134,7 @@ function scm_prompt_info_common {
   fi
 
   # TODO: consider adding minimal status information for hg and svn
+  [[ ${SCM} == ${SCM_P4} ]] && p4_prompt_info && return
   [[ ${SCM} == ${SCM_HG} ]] && hg_prompt_info && return
   [[ ${SCM} == ${SCM_SVN} ]] && svn_prompt_info && return
 }
@@ -193,6 +203,26 @@ function git_prompt_vars {
   SCM_CHANGE=$(_git-short-sha 2>/dev/null || echo "")
 }
 
+function p4_prompt_vars {
+  IFS=$'\t' read -r \
+     opened_count non_default_changes default_count \
+     add_file_count edit_file_count delete_file_count \
+     <<< "$(_p4-opened-counts)"
+  if [[ "${opened_count}" -gt 0 ]]; then
+    SCM_DIRTY=1
+    SCM_STATE=${SCM_THEME_PROMPT_DIRTY}
+    [[ "${opened_count}" -gt 0 ]] && SCM_BRANCH+=" ${SCM_P4_OPENED_CHAR}${opened_count}"
+    [[ "${non_default_changes}" -gt 0 ]] && SCM_BRANCH+=" ${SCM_P4_CHANGES_CHAR}${non_default_changes}"
+    [[ "${default_count}" -gt 0 ]] && SCM_BRANCH+=" ${SCM_P4_DEFAULT_CHAR}${default_count}"
+  else
+    SCM_DIRTY=0
+    SCM_STATE=${SCM_THEME_PROMPT_DIRTY}
+  fi
+
+  SCM_PREFIX=${P4_THEME_PROMPT_PREFIX:-$SCM_THEME_PROMPT_PREFIX}
+  SCM_SUFFIX=${P4_THEME_PROMPT_SUFFIX:-$SCM_THEME_PROMPT_SUFFIX}
+}
+
 function svn_prompt_vars {
   if [[ -n $(svn status 2> /dev/null) ]]; then
     SCM_DIRTY=1
@@ -363,6 +393,11 @@ function git_prompt_info {
   echo -e "${SCM_PREFIX}${SCM_BRANCH}${SCM_STATE}${SCM_SUFFIX}"
 }
 
+function p4_prompt_info() {
+  p4_prompt_vars
+  echo -e "${SCM_PREFIX}${SCM_BRANCH}:${SCM_CHANGE}${SCM_STATE}${SCM_SUFFIX}"
+}
+
 function svn_prompt_info {
   svn_prompt_vars
   echo -e "${SCM_PREFIX}${SCM_BRANCH}${SCM_STATE}${SCM_SUFFIX}"