Add type coercion option

New environment option "typeCoercionFns" accepting an object with functions for coercing values to expected types (typically converting strings to booleans or numbers). The functions' names must match their corresponding type (e.g. "boolean" or "number").

See "Type Coercion Option" test at the end of `tests/tests3.js` for example usage.

This actually implements https://github.com/garycourt/JSV/issues/5.