Support GitHub squash-merge commit parsing

[sermojohn]

GitHub provides squash-and-merge option in pull requests. This option generates a custom commit message by default, which is editable by the user.

Gitmono can support the default github squash-merge commit message, so the raw commit messages are parsed out of a squash-merge commit. The conventional or common formatted commit messages affect the version bump (increment) performed by gitmono release command.