support protocol relative urls

[leftieFriele]

Currently you have to implement a custom validation function in order to use protocol relative urls when using the url format in convict. The validator package supports the option allow_protocol_relative_urls when validating urls.

I know similar requests have been closed (#170, #93]. As far as I can tell adding this option to the validator.isUrl call in convict won't break anything. I'd be happy to create a PR for this if you all think this is a good idea.

[A-312]

With my PR #327 , the url format will be a separated paclage see : https://github.com/A-312/node-convict/tree/nodep/packages/convict-validator