Powerline plain theme refactor

- Split functions and options in two files
- Import common functions from powerline theme
- Add README.md

themes/powerline-plain/powerline-plain.base.bash

@@ -0,0 +1,29 @@
+. "$BASH_IT/themes/powerline/powerline.base.bash"
+
+function __powerline_left_segment {
+  local OLD_IFS="${IFS}"; IFS="|"
+  local params=( $1 )
+  IFS="${OLD_IFS}"
+
+  LEFT_PROMPT+="${separator}$(set_color - ${params[1]}) ${params[0]} ${normal}"
+  LAST_SEGMENT_COLOR=${params[1]}
+}
+
+function __powerline_prompt_command {
+  local last_status="$?" ## always the first
+
+  LEFT_PROMPT=""
+
+  ## left prompt ##
+  for segment in $POWERLINE_PROMPT; do
+    local info="$(__powerline_${segment}_prompt)"
+    [[ -n "${info}" ]] && __powerline_left_segment "${info}"
+  done
+  [[ "${last_status}" -ne 0 ]] && __powerline_left_segment $(__powerline_last_status_prompt ${last_status})
+  [[ -n "${LEFT_PROMPT}" ]] && LEFT_PROMPT+="$(set_color ${LAST_SEGMENT_COLOR} -) ${normal}"
+
+  PS1="${LEFT_PROMPT} "
+
+  ## cleanup ##
+  unset LEFT_PROMPT
+}