Add better out of the box support for the "triangular" workflow

Git 1.8.3 introduced the `remote.pushdefault` option to support the *triangular workflow* that is so common when working with GitHub, and it has been polished recently.

The idea is to push to one remote (your fork) but pull from another one (the blessed repo). So instead of having your fork as the default remote when doing `git hub clone`, we should use the blessed repo instead, and then set the `remote.pushdefault` to the user's fork.

Since this feature is fairly recent, we could introduce a `--triangular` option to the clone command (and a matching `hub.clonetriangular` config variable) to enable this new mode.