support forwarding (and caching) of non GetMap params, e.g a TIME or ELEVATION field.

configuration should include: * list of params that can be forwarded * default value for a param if it is missing * list of values accepted for a param * template string indicating how cached tiles should be stored on disk (e.g. /layer/elevation-100/08/.... , or /layer/elevation/100/08....., etc)

fixed