Simplify ValueMappers

[escowles]

Many of the ValueMapper implementations seem very boilerplate-repetitive. Many of them differ only in a few key details, e.g., the prefix used to identify the value, the method to convert values from Strings.

Could we create a ValueMapper base class or DSL that simplified e.g., a ValueMapper that supported integers, to something like value_mapper :integer, :to_i ?