Added 'APPEND' method

### Summary

This version includes an `APPEND` method, which allows a user to request paths which include illegal Python identifiers.

### Why?

I was using an API at work which included a hyphenated path. I looked online to see how I can do that with Hammock, but as it turns out, there is no documented method. My boss remarked that I can use `getattr`

`getattr(hammock, url)` is possible, but it's not very clean.

### Side note

Thanks a lot for building Hammock. It's wonderful!

I'd be happy to change aspects of this PR if you feel it doesn't fit with your project.