chore(gendocs): add npm run gendocs command

[nfischer]

Now you can generate documentation by running npm run gendocs. Once this is merged, we can add this as a step to the contributing guidelines along the lines of:

Before committing:

• Run npm test to make sure tests & linting pass
• Run npm run gendocs to make sure docs are up to date

Is travis-only failing? Make sure you run npm run gendocs before committing!

[ariporad]

LGTM