plugins/powerline: use `_command_exists`

Addresses bash-it/bash-it#1632

plugins/available/powerline.plugin.bash

@@ -1,9 +1,9 @@
-#!/usr/bin/env bash
+# shellcheck shell=bash
 
 cite about-plugin
 about-plugin 'enables powerline daemon'
 
-command -v powerline-daemon &>/dev/null || return
+_command_exists powerline-daemon || return
 powerline-daemon -q
 
 #the following should not be executed if bashit powerline themes in use
@@ -14,13 +14,13 @@ case "$BASH_IT_THEME" in
 esac
 POWERLINE_BASH_CONTINUATION=1
 POWERLINE_BASH_SELECT=1
-bashPowerlineInit=$(python -c \
+bashPowerlineInit="$(python -c \
 	"import os; \
 	import powerline;\
 	print(os.path.join(os.path.dirname(\
 	powerline.__file__),\
 	'bindings', \
 	'bash', \
-	'powerline.sh'))")
+	'powerline.sh'))")"
 [ -e $bashPowerlineInit ] || return
-. $bashPowerlineInit
+source $bashPowerlineInit