Improve the heuristic for URL recognition in autolink

[Piotrek Koszuliński]

Steps to reproduce

Based on ​https://twitter.com/l0c0luke/status/633684769121832960

Autolink will not recognise youtu.be/cSVuh_8-VQs as a link, hence autoembed will not find it too. Links without protocols are a common case as browsers hide them in the URL bar.

Expected result

We may try to improve the heuristic so it autolinks such URLs. It may happen that it will start autolinking too much, but links can always be removed, so it should not be a big problem as long as it's not happening for typical plain text.

[Jakub Ś]

From what I have checked autolink will only work when you paste http://o2.pl but not when you paste text http://o2.pl text or http://o2.pl www.o2.pl test.com. It only assumes that user pastes a link and nothing else.

I personally think this is too strict and that we should allow more:

1. first case is easy - text http://o2.pl text. We just need to remove ^ $ from pattern
2. Second case I'm not sure - should autolink only recognize links starting with http(s)? or www. as well?

[Jakub Ś]

Other issues for autolink plugin: #13569, #14769, #16685.