Add new clone commands for chaining

[ingydotnet]

Currently you can do powerful chains like this:

git hub repos --raw some-org | git hub clone -

But if a target directory already exists, it will fail.

A solution is to provide these commands:

• git hub clone-or-skip -- Don't clone if exists.
• git hub clone-or-pull -- git pull if exists.
• git hub clone-or-ff -- git pull --ff-only if exists.

[perlpunk]

since we also have the command gist-clone soon, maybe options instead of new commands would make more sense?