Allow using nil as explicit transformer value when creating a Form, to disable default transformer

I believe this used to work, but there wasn't a spec for it.  I'm
guessing the changes made between 0.9 and 0.10 broke it.
Add Forme::RawString class and Forme.raw for creating raw strings, so you don't need to use extend(Forme::Raw)

Code that requires Object#extend at runtime is really bad form,
so add a string subclass that includes Forme::Raw to disable
html escaping.