Adding support for more commands

If you think we need to support another command, you can either make an issue requesting support, or just implement it yourself.

Adding new commands is easy. In `lib/index.ts`, you'll see the implementation of our current commands. If your command is already supported by `wd`, you should be able to get all the information you need from `webdriverio`. Make sure to add tests in `spec/index_spec.ts`, and run `npm test` before you submit.