Implement options_after window option

Created new method config_after_window() that applies relevant
configurations after windows and panes have been created. Currently, it
only knows the `options_after` option.
This is useful for setting `synchronize-panes: on` *after* pane
creation and shell command execution.

Author: sebastianst

tmuxp/workspacebuilder.py

@@ -188,6 +188,8 @@ def build(self, session=None):
             if 'focus' in wconf and wconf['focus']:
                 focus = w
 
+            self.config_after_window(w, wconf)
+
             if focus_pane:
                 focus_pane.select_pane()
 
@@ -308,6 +310,14 @@ def get_pane_start_directory():
 
             yield p, pconf
 
+    """
+    Applies window configurations relevant after window and pane creation.
+    """
+    def config_after_window(self, w, wconf):
+        if 'options_after' in wconf and isinstance(wconf['options_after'], dict):
+            for key, val in wconf['options_after'].items():
+                w.set_window_option(key, val)
+
 
 def freeze(session):
     """Freeze live tmux session and Return session config :py:obj:`dict`.