Allow passing additional options for search

[ZakarFin]

And add common 'limit' option to request less or more results than instance default. The instance administrator can restrict the amount of results with a hard limit for searches. Related config in oskari-ext.properties:

# default limit for search results (this works like before)
search.max.results=100
# new option: hard limit for results regardless what client requests (defaults to 10x max results)
search.max.results.hardlimit=1000 

The parameters sent by the client are accessible in search channels so they can have special implementation specific handling based on the additional options/params.