Replace or merge iD data at runtime

[bhousel]

related to osmlab/osm-planning#2

It would be useful to be able to replace the imagery and presets based on a config file passed in via the url params.

Ideally we would support both "replace" and "merge" for these.

http://example.com/#imagery=whatever.json
http://example.com/#mergeImagery=whatever.json

http://example.com/#presets=whatever.json
http://example.com/#mergePresets=whatever.json

cc @kriscarle @geohacker