gh: Provide option to build `hub` without autoupdate

Use Go build tag to build with/without autoupdate. For Homebrew, we build with `./script/build noupdate` and let it handle the update. We could also build with `./script/build` which has autoupdate built-in.

/cc @mislav 
You really hit the point! I do want to abstract configs into one place. Doing it right now will break the schema of `~/.config/hub`. I'd suggest to migrate after the 2.0 is released along with [the `ini` format changes](https://github.com/github/hub/pull/621#issuecomment-55606637). We'll disable autoupdate (build with `./script/build noupdate`) for 2.0 release for now.

What do you think?