Allow file:/// protocol in links

[mtmacdonald]

In plain HTML, I can link to a local file using the file:/// protocol, like so:

 <a href="file:///C:/test.docx">Link</a>

I would expect the equivalent CommonMark link to be expressed like this:

 [Link](file:///C:/test.docx)

I can't see anything in the CommonMark spec that prohibits the file:/// protocol from being used in links, and indeed the reference implementation parses this as expected:

However the markdown-it implementation does not:

Is this a bug?

[puzrin]

https://github.com/markdown-it/markdown-it/blob/master/lib/index.js#L285

That's a security restriction to be safe by default. Override it if needed.