Allow specifying git branch of extensions

[shadyvb]

Resolves #852

I don't really have Ruby experience, but this does seem to work. Happy to update/tweak as needed.

[shadyvb]

Another reason for adoption to this is the tester extension being broken when it is updated while project codebases are not, just hit this myself, had to revert the extension for phpunit to work.

[BronsonQuick]

@shadyvb Nice work Sir. This looks good to me. Can you please update the documentation to cover this change as well please then I'm happy to test it and merge it. Thank you!

[BronsonQuick]

I was having another think about this PR this morning and I am wondering if instead of the -b branch-name, it might be better using the commit hash?

[shadyvb]

@BronsonQuick this is a bit more restrictive, if I'm specifying a branch name I'd probably want to receive updates to it too.

Maybe we can support both ? repo@branch and repo#commit ? a bit more logic to add though, as it'll require a second conditional git checkout call.

[BronsonQuick]

@shadyvb Supporting both sounds good to me Sir! I've not been one to maintain branches as it doesn't take long for branches to diverge and not be maintained. I'm happy for us to support both though and other contributors to extensions can maintain the branches.