[BUG] macOS: Markdown not rendered in notifications

Keybase notifications on macOS via the notification center display Markdown as is – which is particularly uncool with Keybase own's managed bots like reminderbot, githubbot etc:

Is it possible to support at least part of the Markdown and ignore/strip the rest?

In the above example, `:bell:` would work if it were translated into the corresponding UTF character beforehand.

(As an alternative, bots could use UTF instead of `:whatever:` directly, but that seems to be the wrong approach IMO.)

Reproduce the issue in the macOS notification center using a notification containing Markdown and emoji-style codes such as `:bell:`. Locate the macOS notification implementation, then determine the supported rendering or stripping behavior; done means notification text no longer displays raw Markdown and the requested emoji conversion works as intended.

Written by the indexing model from the issue text.