Add run_binding function to bindings.h.
> Runs the given binding and handles parse errors. Returns a
> CommandResult for running the binding's command. Caller should render
> tree if needs_tree_render is true. Free with command_result_free().
e.g. pressing Mod1+x when having the following in your configfile:
bindsym Mod1+x some invalid command
will lead to an i3-nagbar instance popping up, offering you to view the
error log (which will contain parser errors from this commit on).
