Add Filters, Aliases

[martindale]

Resources should be filterable, allow for aliases of these filters.

Proposal:

maki.define('Play', {
  attributes: {
    state: { type: String , enum: ['queued', 'playing', 'played', 'skipped'], default: 'queued' }
  },
  filters: {
    'playing': { state: 'playing' }
  }
}

...would add a new endpoint at /plays/playing that serves only documents with state "playing", identical to a query of /plays?state=playing.

Not sure if this is a good idea, as it doesn't "feel" pure. Feedback appreciated.