Allow setting a commit message prefix

[shreyasminocha]

Feature ✨

gitupdate -p "Update " . # `Update foo bar baz`

gitupdate --prefix "Update " .

Why?

I have a global githook that, among other things, verifies if the commit message begins with an uppercase letter. In the absence of this option, gitupdate doesn't work with my githook (it emits an obscure error message).

[rnhurt]

I think it would be handy, in my use case, to prefix the commit with "WIP: ".