feat: custom merge commit messages

[AndersDJohnson]

Allow configuration of custom merge commit messages, possibly through a custom plugin function/module. Enable to override the default squash merge commit message behavior of rolling up the PR's commits' messages in a bulleted list with the PR title at the top. E.g., to pull from the PR's body/description, labels, etc.

[AndersDJohnson]

#3 introduces another option, but still not totally custom.

[AndersDJohnson]

Implemented option to use PR title as commit message in 9ddbb77 - still not totally custom.