Remove "$()" method in pre-prompt hook

Using "$()" method on pre-prompt hook is just asking for security
trouble.

powerline/bindings/bash/powerline.sh

@@ -1,8 +1,3 @@
-_powerline_prompt_command() {
-	export PS1="$(powerline-prompt --last_exit_code=$? left)"
-	_powerline_tmux_set_pwd
-}
-
 _powerline_tmux_setenv() {
 	if [[ -n "$TMUX" ]]; then
 		tmux setenv TMUX_"$1"_$(tmux display -p "#D" | tr -d %) "$2"
@@ -20,4 +15,5 @@ _powerline_tmux_set_columns() {
 trap "_powerline_tmux_set_columns" SIGWINCH
 kill -SIGWINCH "$$"
 
-export PROMPT_COMMAND="_powerline_prompt_command"
+export PROMPT_COMMAND="_powerline_tmux_set_pwd"
+PS1='$(powerline-prompt --last_exit_code=$? left)'

powerline/renderers/shell.py

@@ -36,3 +36,7 @@ class ShellRenderer(Renderer):
 				if attr & Renderer.ATTR_BOLD:
 					ansi += [1]
 		return '[{0}m'.format(';'.join(str(attr) for attr in ansi))
+
+	@staticmethod
+	def escape(string):
+		return string.replace('\\', '\\\\')

powerline/renderers/zsh_prompt.py

@@ -15,4 +15,4 @@ class ZshPromptRenderer(ShellRenderer):
 
 	@staticmethod
 	def escape(string):
-		return string.replace('%', '%%')
+		return string.replace('%', '%%').replace('\\', '\\\\')