[commands] Split process_commands into lower level bits.

discord/ext/commands/context.py

@@ -117,6 +117,11 @@ class Context(discord.abc.Messageable):
         ret = yield from command.callback(*arguments, **kwargs)
         return ret
 
+    @property
+    def valid(self):
+        """Checks if the invocation context is valid to be invoked with."""
+        return self.prefix is not None and self.command is not None
+
     @asyncio.coroutine
     def _get_channel(self):
         return self.channel