Support for nested params

[shhavel]

This is related to closed issue #33
Sometimes this is really needed, e.g. if search parameters are nested by API design.
Please consider.

[mattt]

Thanks for your contribution, @shhavel. I think nested parameters are something that make sense for the next major version of sinatra-param. As far as implementations go, I think I prefer the nested version in #60, and will likely go that route instead.