Add --consider-git-uris-safe option to CLI.

This allows using bundler-audit for Gemfiles using the `github:` shorthand syntax.

Bundler expands `github:` to `git://github.com`. A pull request changing this to `https:` has not been accepted, as the developers of bundler do not consider `git:` as inherently unsafe: https://github.com/bundler/bundler/pull/1820

PS: I'm not 100% sure about the name of the option – if anyone comes up with a better name I’ll be happy to change it :smiley: