Add `search` or `filter` method

[martinfleis]

We have over 200 providers at this moment and it may be helpful to have a function that is able to filter providers based on some criteria. I would imagine keywords (e.g. I want all HERE maps) or specs (only those that do not need a token).

API-wise, maybe something like:

Bunch.filter(keyword=None, name=None, requires_token=None...) # maybe some other (zoom, bounds?)

It would return a smaller Bunch or an individual tile provider if name==TileProvider.name.