Decode colon-sequences in Love sent from Slack

[bwbaugh]

At display time, Slack clients are encouraged to convert these colon-sequences into native Emoji where available…

See the docs: https://api.slack.com/docs/message-formatting#emoji

This is not an ask to support custom emoji, but just the standard Unicode set. We should be able to send a character like🦊 and have it rendered as that instead of being rendered as :fox_face: in the Love text.

[xiaominmalia]

It's good