Preserve triple hyphens etc. in URLs.

[bep]

This relates to #7288 -- which was fixed.

I create this just to avoid some yelling about "broken URLs" from @kaushalmodi and friends.

I'm doing some ground work to simplify stuff and also make stuff faster and more effective.

So with #7288 the issue was

content/foo--bar.md, the page URL is expected to be /foo--bar/.

With that it does not make much sense to say that content/foo---bar.md becomes foo-bar/.

We have a test case for the above, but I will change that so that any number of subsequent hyphens are preserved...

[kaushalmodi]

With that it does not make much sense to say that content/foo---bar.md becomes foo-bar/.

Certainly! If the user decided to have 3 hyphens in their content file name, then why override that.