config: add support for appending a 'spawn' argument to key bindings

A key binding may now have an optional ':<cmd>' string appended to the
key.

This is intended to be used like so:

  pipe-scrollback:sh -c "cat > file"=Print

TODO: we still only allow one *action*. Meaning you still cannot
specify multiple pipe-scrollback bindings, for example.

config.h

@@ -49,6 +49,8 @@ struct config {
     struct {
         /* Bindings for "normal" mode */
         char *key[BIND_ACTION_COUNT];
+        char *spawn[BIND_ACTION_COUNT];
+
         struct mouse_binding mouse[BIND_ACTION_COUNT];
 
         /*