[gh] Go powered Hub 2.0

As discussed in https://github.com/github/hub/issues/475,  the next major release of hub will be Go-powered by merging in [jingweno/gh](https://github.com/jingweno/gh). It's been so long :smile_cat: 

Some remaining works:

1. [ ] Merge gh branch to master
2. [ ] Clean up the Ruby-related implementation after this merge
3. [ ] Fix remaining bugs, e.g., https://github.com/github/hub/issues/636
4. [ ] Make a preview release for `hub` 2.0

/cc @mislav