Potential Feature: Integrate Snazzy Maps

I've always found the custom styling within ggmap to be a bit complicated, and I feel it is an underutilised feature because of this. While CloudMade used to offer some functionality in using custom styles, this has been defunct for a long while.

I have built a package, ggmapstyles, which makes it really easy to select custom designs on the Snazzymaps website. It is hosted on GitHub here: https://github.com/mikey-harper/ggmapstyles

I have added some basic functions including `get_snazzymap`, and the package scrapes the JSON styling directly from the URL of the specified Snazzymap page like this: https://snazzymaps.com/style/93/lost-in-the-desert

The documentation is not complete, but before I put too much effort into making it a standalone package, I was wondering whether this should be integrated directly into ggmap? I am happy to develop it further though and release it as its own thing, or at least develop it as a separate branch for now before potentially merging in the future.

I have included basic guidance with the package on how to use it, so feel free to try it out (hopefully it should be stable!).

P.S. my first try at building a proper package so hopefully, it all works :D

Mikey