Add URL validator

[vascoguita]

Closes #224

[mildebrandt]

Hi! I'm no longer the maintainer, but a few things stood out:

• In the past, we've worked to keep the dependencies down to just the yaml related packages. Adding the requests package would be unnecessary for the majority of yamale users.
• The only protocols supported are http and ftp...it would be good to see all possible protocols supported.
• Your functional test isn't running. See test_data and test_bad_subset2() in test_functional.py for an example.
• The regex doesn't accept some URLs....such as http://xn--nw2a.xn--j6w193g/

Good luck getting this in.

[vascoguita]

Hello @mildebrandt, thank you for your feedback. I'll try to sort out the points you've mentioned and hopefully we can have this merged soon :)

[vascoguita]

Hello again @mildebrandt, It's possible to check if a URL is reachable using just the Python Standard Library (without requests). However it will be hard to add a URL validator that accepts all possible URLs without bloating Yamale with a big regular expression or without adding a dependency such as validators.

I think, a more satisfactory solution is to use this URL validator as a custom validator, without merging it to Yamale. I'm therefore closing this PR.