WMS GetMap support of extended parameters

[pomadchin]

This is a follow up task for the #235 to add support of such extended parameters in the GetMap function.

I see some potential complexity in overriding values of a MAML expression from the application.conf.
I added an extra point to investigate this problem. If neccesary a follow up issue would be created.

[pomadchin]

Checkout new issues under the epic https://github.com/azavea/geotrellis/issues/235