Decouple dependency on CLI gh

[rnorth]

For expediency the tool currently execs gh as a subprocess for several operations with GitHub.

We should break this dependency so that turbolift has no dependencies on external programs.

[kurtmckee]

I chose turbolift over other tools precisely because it simply used gh under the hood.

I would advocate to use gh if it's available and fall back to something else -- and the additional configuration that entails -- if gh is unavailable.