Add prompter option to append .gitmessage to commits

[thesheps]

Hey folks! Not sure if this is within the guidelines of the community, but thought it would be a useful extension to allow whatever's already been baked into the .git/.gitmessage to be appended to the end of a conventional commit?

Common usecases include things like git-mob which appends co-authors to this file automatically.

#152 addresses this, would love to start a conversation about it if folks find this useful!