Adding optional "converter" function for validating/converting types.

[totherik]

I apologize if this has been discussed in the past (couldn't find a related issue), but just filing this to see if there's an appetite for some version of this functionality. If so, I'm happy to do the work.

The proposal is to add convert support to the type validator by adding an optional converter function to the signature: type(constructor, [name], [converter]);

The converter function would have the form:

Joi.type(CustomType, function (value) {
    return new CustomType(value);
});

A sample back-of-napkin implementation may look like:
SAMPLE REDACTED

Edit: Sample code was woefully incorrect and out of date. Working branch is here.

[totherik]

NOTE: in the above example the 'converter' variable should default to a noop identity function.

Edit: Back near keyboard, so updated sample code to reflect this change.
Edit 2: Realizing I've overlooked some fundamental behavior in Joi such that this isn't anywhere near the behavior that would have to change for conversion to work correctly. Still interested in hearing if this feature has legs, though, so will dig further.

[Marsup]

Don't go too far into this feature, it might be done otherwise.

[totherik]

Yeah, no worries. Just had some time to work on it so I put together a PR, but I won't be heartbroken if it gets closed.

[Marsup]

Please look at #577 and see if it covers your needs. I'll close the PR as well.

[lock]

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