theme/powerline-plain: adopt automatic history

Use `preexec` and `$HISTCONTROL` to configure _Bash It_'s automatic history management feature.

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

@@ -1,5 +1,18 @@
+# shellcheck shell=bash
 . "$BASH_IT/themes/powerline/powerline.base.bash"
 
+case $HISTCONTROL in
+*'auto'*)
+	: # Do nothing, already configured.
+	;;
+*)
+	# Append new history lines to history file
+	HISTCONTROL="${HISTCONTROL:-}${HISTCONTROL:+:}autosave"
+	;;
+esac
+safe_append_preexec '_bash-it-history-auto-load'
+safe_append_prompt_command '_bash-it-history-auto-save'
+
 function __powerline_left_segment {
   local OLD_IFS="${IFS}"; IFS="|"
   local params=( $1 )
@@ -34,8 +47,6 @@ function __powerline_prompt_command {
   LAST_SEGMENT_COLOR=""
   PROMPT_AFTER="${POWERLINE_PROMPT_AFTER}"
 
-  _save-and-reload-history "${HISTORY_AUTOSAVE:-0}"
-
   ## left prompt ##
   for segment in $POWERLINE_PROMPT; do
     local info="$(__powerline_${segment}_prompt)"