Add last resort CSV decoder for slice types

This adds support for requests like "bar?foo=1,2,3" where the field foo is an []int.
For longer lists this is easier to work with and understand than "bar?foo=1&foo=2&foo=3..."

This should work for most array types.
The only issue I can think of off hand is the []string type.
Do should it break &foo=a,b,c&foo=d into ["a","b","c","d"] or ["a,b,c", "d"]?
For now it is set to the second option which should not break any backwards compatability.