Add support for personal metadata repos

[imsky]

right now pull-review is optimized for monorepos, where all the notification metadata lives in the same repo as the code. this means that it must be duplicated if pull-review is added to another repo in the organization, and the data can thus become stale.

one way around this is a personal notification metadata repo, like imsky/pull-review-notification with a single file inside it that specifies the Slack, etc. handles to use for the user (can also specify the Slack workspace).