Create VimPowerline.reset_highlight proxy method

Fixes #411

powerline/bindings/vim/plugin/powerline.vim

@@ -82,7 +82,7 @@ function! PowerlineRegisterCachePurgerEvent(event)
 endfunction
 
 augroup Powerline
-	autocmd! ColorScheme * :exec s:powerline_pycmd 'powerline.renderer.reset_highlight()'
+	autocmd! ColorScheme * :exec s:powerline_pycmd 'powerline.reset_highlight()'
 	autocmd! VimEnter    * :redrawstatus!
 	autocmd! VimLeavePre * :exec s:powerline_pycmd 'powerline.shutdown()'
 augroup END

powerline/vim.py

@@ -82,3 +82,14 @@ class VimPowerline(Powerline):
 	@staticmethod
 	def get_segment_info():
 		return {}
+
+	def reset_highlight(self):
+		try:
+			self.renderer.reset_highlight()
+		except AttributeError:
+			# Renderer object appears only after first `.render()` call. Thus if 
+			# ColorScheme event happens before statusline is drawn for the first 
+			# time AttributeError will be thrown for the self.renderer. It is 
+			# fine to ignore it: no renderer == no colors to reset == no need to 
+			# do anything.
+			pass