Introduce GlobalSettings.Features()

Allow consumers to see what features are included with the libgit2 build that is being used.

Right now we do a little scrubbing in our UI so that we display nicer error messages when somebody tries to clone an `ssh://` URL, since we do not yet support SSH.  (Instead of saying "This version of libgit2 was not built with SSH support", which is some very leaky abstraction.)  If somebody were to bring their own libgit2 to the party, though, it would be nice if we would *not* do this, and instead just pass the URL right down to libgit2.

Thus we would like to query the built features.