Copied command_exists function to the helpers lib so that there is no hidden dependency on the base plugin

The original command_exists function will stay in the base plugin, but will no longer be used by other plugins or themes.

themes/atomic/atomic.theme.bash

@@ -164,7 +164,7 @@ ___atomic_prompt_clock() {
 }
 
 ___atomic_prompt_battery() {
-  ! command_exists battery_percentage ||
+  ! _command_exists battery_percentage ||
   [ "${THEME_SHOW_BATTERY}" != "true" ] ||
   [ "$(battery_percentage)" = "no" ] && return
 

themes/base.theme.bash

@@ -474,7 +474,7 @@ function battery_char {
     fi
 }
 
-if ! command_exists battery_charge ; then
+if ! _command_exists battery_charge ; then
     # if user has installed battery plugin, skip this...
     function battery_charge (){
 	# no op
@@ -484,7 +484,7 @@ fi
 
 # The battery_char function depends on the presence of the battery_percentage function.
 # If battery_percentage is not defined, then define battery_char as a no-op.
-if ! command_exists battery_percentage ; then
+if ! _command_exists battery_percentage ; then
     function battery_char (){
 	# no op
 	echo -n

themes/brainy/brainy.theme.bash

@@ -152,7 +152,7 @@ ___brainy_prompt_clock() {
 }
 
 ___brainy_prompt_battery() {
-	! command_exists battery_percentage ||
+	! _command_exists battery_percentage ||
 	[ "${THEME_SHOW_BATTERY}" != "true" ] ||
 	[ "$(battery_percentage)" = "no" ] && return
 

themes/demula/demula.theme.bash

@@ -84,7 +84,7 @@ ${D_BRANCH_COLOR}%b %r ${D_CHANGES_COLOR}%m%u ${D_DEFAULT_COLOR}"
 
 # checks if the plugin is installed before calling battery_charge
 safe_battery_charge() {
-  if command_exists battery_charge ;
+  if _command_exists battery_charge ;
   then
     battery_charge
   fi

themes/powerline/powerline.base.bash

@@ -43,9 +43,9 @@ function __powerline_user_info_prompt {
 function __powerline_ruby_prompt {
   local ruby_version=""
 
-  if command_exists rvm; then
+  if _command_exists rvm; then
     ruby_version="$(rvm_version_prompt)"
-  elif command_exists rbenv; then
+  elif _command_exists rbenv; then
     ruby_version=$(rbenv_version_prompt)
   fi
 

themes/rana/rana.theme.bash

@@ -117,7 +117,7 @@ ${D_BRANCH_COLOR}%b %r ${D_CHANGES_COLOR}%m%u ${D_DEFAULT_COLOR}"
 
 # checks if the plugin is installed before calling battery_charge
 safe_battery_charge() {
-  if command_exists battery_charge ;
+  if _command_exists battery_charge ;
   then
     battery_charge
   fi