Retain spaces in quoted strings in window options

Fixes tmux-python/tmuxp#239

With the below in tmux config:

    setw -g pane-border-format " #P "

Before the fix:

    ('pane-border-format', '"', '#P', '"')

After the fix:

    ('pane-border-format', ' #P ')

Author: kaushalmodi

libtmux/window.py

@@ -9,6 +9,7 @@
 
 import logging
 import os
+import shlex
 
 from . import exc, formats
 from .common import TmuxMappingObject, TmuxRelationalObject
@@ -190,7 +191,10 @@ def show_window_options(self, option=None, g=False):
                 *tmux_args
             ).stdout
 
-        cmd = [tuple(item.split(' ')) for item in cmd]
+        # The shlex.split function splits the args at spaces, while also
+        # retaining quoted sub-strings.
+        #   shlex.split('this is "a test"') => ['this', 'is', 'a test']
+        cmd = [tuple(shlex.split(item)) for item in cmd]
 
         window_options = dict(cmd)