Support a `danger init` via `yarn create`

[orta]

yarn create danger could run through the sort of thing inside danger init. Note that danger init should probably re-use docs from inside the source code using JSDoc, sooo it'll be a little bit tricky to get off the ground, but considering how often it's out of date vs the website it's better in the long run.

[orta]

You can see the ruby version of danger init here - it's basically a long wizard: https://github.com/danger/danger/blob/master/lib/danger/commands/init.rb#L28

[orta]

danger init now exists, so this might be feasible.