Adds assertion for URLs

[tomaszhanc]

It validates the URL using filter_var. The validation is not so strict to accept only http or https protocols but it might be good. It's definitely lighter than using that monster regex I initially used in that PR.

[zerkms]

I don't generally agree that something as this should be added to the library, but I'm not the decider.

What I want to note though is: I believe we should accept mixed instead @param string $value. Relevant: #197 (comment)

[tomaszhanc]

I changed the type to mixed as adjusted the code after rebasing with the master branch. I'm still not sure if there is something I could do to improve it. If there is, please let me know. If it's not and it's not going to be merged than let's close it, no hurt feelings :)