Add type coercion option

[sgoumaz]

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.

[sgoumaz]

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