add URL validator implementation & tests

[bieli]

Feature request #213 implementation.

[mildebrandt]

Hi! I have a few concerns about using the validators package:

1. It's still pre 1.0, so things can break between minor versions. Why isn't http://localhost:PORT a valid URL? python-validators/validators#285 (comment)
2. It doesn't support http://localhost by default. url validator rejects some local URLs as invalid even though they should be valid python-validators/validators#392
3. It doesn't support query strings without values. [Question]: Are URL query strings containing keys without values invalid on purpose? python-validators/validators#363
4. The supported schemes are hard coded. https://github.com/python-validators/validators/blob/c9585e91f8b409029b059df148da4dc52bd951e3/src/validators/url.py#L42

And that's what I found in just 10 minutes. I'm sure this could work for a subset of users....but it's not a generic solution for all valid URLs.

[bieli]

Sure. Thanks for this feedback. I'll find something more universal.

[mildebrandt]

Cool. FYI, currently Yamale has only one dependency....pyyaml. Anything you add will double the number of dependencies. I don't know how the current maintainers feel about that, but just something to keep in mind.

A couple other things:

1. Don't update the version of Yamale in your PR. That's done during the release process.
2. Add any new dependencies in the setup.py file.

Good luck!