support bot commands in comments

[MartinNowak]

Interestingly Gitlab also supports some "bot" commands in the comment field, e.g. /wip to toggle WIP.
dlang-bot could delete such comments after execution in order to reduce spam on PRs.

Preview would require a custom WebExtension plugin though :/.

[CyberShadow]

For the WIP case I understand it's no longer relevant as GitHub now has draft PRs.

What other commands could the bot understand?