Add option to allow formatting files listed in .gitignore

[JohnnyMorganz]

Closes #895

Not sure what the best name for this is, I've gone with --allow-gitignore for now

[ValdezFOmar]

Not sure what the best name for this is, I've gone with --allow-gitignore for now

May I suggest --no-ignore as an alternative? At least fd and ripgrep use that flag with this meaning.