Add git clone command

[Macavirus]

Partially addresses #48

This makes no actual changes to any commands, but adds the ability to call Clone function.

As there is still some ambiguity around how we should address the actual turbolift clone command, I will address that in a followup PR.

@sledigabel @rnorth would you prefer to preserve current behaviour in clone.go to fork by default? I think not forking by default is more user-friendly in general, but it sounds like the Skyscanner use case requires bulk forking.

My suggestion would be to make the new Clone command default, and place ForkAndClone behind a different cobra command.

[sledigabel]

Your suggestion makes a lot of sense and I don't have any objection for this to be the default behaviour.
On the other hand I don't think forkAndClone should be a separate command, since it's partially the behaviour of clone.
Perhaps a simple flag such as --fork to make the behaviour explicitly forking?

[rnorth]

Argh I should have put this comment here. But yes, I agree with @sledigabel!

[rnorth]

I think this looks fine to me if we’re just implementing the ‘back half’ of the feature. I’d suggest, though, doing the modifications to the clone cobra command as well.

[sledigabel]

Agreed with @rnorth, let's add the whole thing into this PR rather than breaking it down into multiple PRs as we have agreed this will be the new default.

[sledigabel]

@Macavirus how do you want to proceed? do you want to amend this PR and change the default behaviour of clone? I'm happy to add on to your PR if you're swamped, this feature would be a great addition.

[rnorth]

@sledigabel shall we discuss one of us picking this up some time?

[sledigabel]

GMTA, I have a draft for this :-) publishing soon

[sledigabel]

Merging this and will iterate from there.

Thanks a lot for your contribution @Macavirus !