Add zsh prompt renderer

Zsh can use regular escape sequences in the prompt, but they must be
wrapped in %{...%} to work correctly.

Refs #84.

powerline/renderers/zsh_prompt.py

@@ -0,0 +1,14 @@
+# -*- coding: utf-8 -*-
+
+from powerline.renderers.shell import ShellRenderer
+
+
+class ZshPromptRenderer(ShellRenderer):
+	'''Powerline zsh prompt segment renderer.'''
+	def hl(self, fg=None, bg=None, attr=None):
+		'''Highlight a segment.
+
+		Returns the default ShellRenderer escape sequence with %{...%} wrapped
+		around it (required in zsh prompts).
+		'''
+		return '%{' + super(ZshPromptRenderer, self).hl(fg, bg, attr) + '%}'