Allow more relative links in safe mode (issue #517)

[Crozzers]

This PR fixes #517 by expanding the scope of the _safe_href regex to include more types of relative links.

Previously, #513 was merged which allowed the following kinds of relative links:

[issue1](./issue1)
[issue1](/issue1)

But did not allow this: [link](issue1)

The new regex should allow URLs to omit the protocol section of the URL or use relative paths instead of a protocol (eg: ./, ../, /) followed by a hostname, optional port number and then the rest of the URL.

I've also expanded the number of accepted protocols to include mailto: and tel:.

Also, the _safe_protocols attribute has been re-introduced to allow users to extend the number of allowed protocols when operating in safe mode (see this comment).

[nicholasserra]

@Crozzers wanted to check with you on this before I merge and release. Do you think this PR is done and ready to go?

[rouilj]

In message ***@***.***>, Nicholas Serra writes:

@Crozzers wanted to check with you on this before I merge and release. Do you think this PR is done and ready to go?

I would say yes, it's good to go. It fixes my initial use case.

…

-- -- rouilj John Rouillard =========================================================================== My employers don't acknowledge my existence much less my opinions.

[Crozzers]

Yep, good to go