Wrap set_size in sh

Some shells may be weird and add some extra escape codes.
Let's just force this to be run with sh to get rid of these external
variables outside of our control.

We had a similar problem in #27 already and now it occurs in a different
form.

Fixes #52

Author: vimpostor

autoload/tpipeline/util.vim

@@ -12,7 +12,7 @@ func tpipeline#util#left_justify(str)
 endfunc
 
 func tpipeline#util#set_size()
-	let g:tpipeline_size = str2nr(systemlist("tmux display-message -p '#{window_width}'")[-1])
+	let g:tpipeline_size = str2nr(systemlist("sh -c 'tmux display-message -p \"#{window_width}\"'")[-1])
 endfunc
 
 func tpipeline#util#check_gui()