Split off window_width message from shell output

We have to explicitly print a newline to improve robustness.
This is an addendum to 5f663e8 and
should help with #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("sh -c 'tmux display-message -p \"#{window_width}\"'")[-1])
+	let g:tpipeline_size = str2nr(systemlist("sh -c 'echo \"\"; tmux display-message -p \"#{window_width}\"'")[-1])
 endfunc
 
 func tpipeline#util#check_gui()