[core/input] Invoke commands in a subshell

add shell capability to util.cli and make sure that the input module
uses that to reliably spawn whatever command the user wants to run.

see #628

bumblebee_status/core/input.py

@@ -38,7 +38,7 @@ def __event_id(obj_id, button):
 
 def __execute(cmd):
     try:
-        util.cli.execute(cmd, wait=False)
+        util.cli.execute(cmd, wait=False, shell=True)
     except Exception as e:
         logging.error("failed to invoke callback: {}".format(e))