Add strategy (desktop/mobile) parameter

Hey @blahed, I come in peace! Just want to say that this is a fantastic library and thanks for taking the time to build this.

The purpose of my pull request is to add an extra command line parameter for designating which **strategy** (`desktop` or `mobile`) is used when analyzing the page with PageSpeed Insights:

https://developers.google.com/speed/docs/insights/v1/getting_started#api_params

In the process of implementing this feature, I also decided to update the command line interface to parameterize the url (`-u`, `--url`) for consistency.

There are still a few more tasks I want to complete until this is *done*:

- [ ] Add unit tests
- [ ] Bump `VERSION`? (since this changes the interface, i'm guessing `0.1.0`?)

Let me know if you have any feedback or questions!