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
Update only if segment_info is not None
Merge remote-tracking branch 'zyx-i/fix-391' into develop