Breaking change: All the entities with an `id` property have String ids, but all
endpoints that take ids used `u64` ids. This changes that so that all
the methods that take ids, take them as `&str`.
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