add support for 'silent' mode

[boegel]

By default, the current implementation of the bot is very chatty:

• It reports for duty in every PR being opened;
• It acknowledges receiving a command, even when that command is not relevant to that instance;
• It adds a comment if it takes any action in response to a command;

Now that we have various bots running, a lot of semi-useless comments are being created.
As long as the bots are active and listening, those comments are basically always ignored.

So I think we should add support to the bot for staying silent unless asked to reveal itself in PRs and not to reply to every command it receives. It should always post a comment when it actually acts on a command though, of course.

I would argue that this "silent mode" should be enabled by default even, and that bot instances should either be configured to be chatty, or that commands should enable "verbose", which could be useful for debugging purposes.