Add some options for mapping POCO fields into a LiquidHash via .ToLiquid()

[mikebridge]

Right now it calls .ToLower on everything.

[schotime]

Where is this one up to?

We have a branch that we're using https://github.com/mikebridge/Liquid.NET/compare/master...schotime:case_insensitive_liquid_hash that just does the hash, but it obviously doesn't handle the local variables.