Make Tmux and PangoMarkup Renderers merge arguments like shell one

For PangoMarkup it is pretty useless currently (segment_info does not contain 
anything useful).

Note: this does its job by replacing default behavior.

Source of issue: previous default used either segment_info argument (containing 
{"args": args, "environ": os.environ}) or default segment_info, shell renderer 
merged default segment_info with .render() argument. Now segment_info is merged 
by default and old behavior moved to vim renderer which is the only one that is 
designed to use this.

Fixes #391

powerline/renderer.py

@@ -67,7 +67,11 @@ class Renderer(object):
 		return segment
 
 	def get_segment_info(self, segment_info):
-		return segment_info or self.segment_info
+		r = self.segment_info.copy()
+		r.update(segment_info)
+		if 'PWD' in r['environ']:
+			r['getcwd'] = lambda: r['environ']['PWD']
+		return r
 
 	def render(self, mode=None, width=None, side=None, output_raw=False, segment_info=None, matcher_info=None):
 		'''Render all segments.

powerline/renderers/shell.py

@@ -19,13 +19,6 @@ class ShellRenderer(Renderer):
 	tmux_escape = False
 	screen_escape = False
 
-	def get_segment_info(self, segment_info):
-		r = self.segment_info.copy()
-		r.update(segment_info)
-		if 'PWD' in r['environ']:
-			r['getcwd'] = lambda: r['environ']['PWD']
-		return r
-
 	def hlstyle(self, fg=None, bg=None, attr=None):
 		'''Highlight a segment.
 

powerline/renderers/vim.py

@@ -65,6 +65,9 @@ class VimRenderer(Renderer):
 			def strwidth(string):
 				return vim.strwidth(string)
 
+	def get_segment_info(self, segment_info):
+		return segment_info or self.segment_info
+
 	def render(self, window_id, winidx, current):
 		'''Render all segments.'''
 		if current: