This adds the 5 methods for the mastodon API that deal with keyword
filtering:
GET /api/v1/filters
POST /api/v1/filters
GET /api/v1/filters/:id
PUT /api/v1/filters/:id
DELETE /api/v1/filters/:id
Closes#71
Need to figure out a better solution for that to_querystring test, it'd
be nice to have something generate every possible permutation of those
builder methods