rename "github_token" to "github-token"

According to [npm coding style](https://github.com/npm/npm/blob/master/doc/misc/npm-coding-style.md#case-naming-etc), config keys are separated by dashes, not underscores.

The point is: if this feature will ever make it into upstream, config key will be named according to that standard. If that happens, I wouldn't want people to change their configs everywhere...

PS: also added some docs, I know nobody cares, but `test/tap/config-meta.js` fails :P