Feature request: ability to add custom text to the message.

[vitalykarasik]

Predefined fields like GitHhub repo, event, workflow are fine, but IMHO we should have ability to add some custom text.
BTW, commit message should part of the notification too.

[gregorybchris]

+1 to this feature ask. Parity with rtCamp/action-slack-notify would be nice.