Update `MR::TypeConverter` to support expanded types

This updates the `MR::TypeConverter` to support expanded string
types. This lines it up with what MR's factory can generate. This
allows more expressive random read model data using factories.
The factories will use these types when generating random data and
the read model will still properly type cast values for fields
using the new types as strings.

Closes #188 

@kellyredding - Ready for review.