[core/input] Execute commands in background

When spawning a command from an input interaction, do it in the
background, so as to not block further interactions.

see #23

bumblebee/input.py

@@ -98,6 +98,6 @@ class I3BarInput(object):
         if callable(cmd):
             cmd(event)
         else:
-            bumblebee.util.execute(cmd)
+            bumblebee.util.execute(cmd, False)
 
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