brew tap: allow tapping non-GitHub repositories.

`brew tap` restricts users to GitHub and checks for private repos. I
created `brew any-tap` to support a wider variety of taps.

With very small changes (and no extra flags!), this change allows users
to use `brew tap` with GitHub as they always have or add one extra
argument and tap any repository of any type from anywhere.

    brew tap user/name              # Same as it ever was
    brew tap user/name URL          # Tap URL, whatever it happens to be

Closes Homebrew/homebrew#40326.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>