improved getURL method

For issue https://github.com/esgibter/fandomstats/issues/6, this change:
* Creates class AO3url, which is a tool that takes in search parameters as an object and spits out a properly-formed AO3 search URL. The getUrl() method can take in any complexity of parameters available (or, at least, whatever complexity I covered in the tests I wrote, which I think is the majority of cases)
* Adds tests for AO3url, which tests for a simple, medium complexity, and high complexity search parameters

The class and method aren't consumed yet - I'm going to do that in my next pull request. That will also include how we store the params object and pass to the backend, which I'm still trying to figure out.