Slugify section anchors

[dtolnay]

For a section like ## Hello World mdBook currently creates an anchor like page.html#Hello World. The anchor should be slugified to be more URL-friendly: page.html#hello-world.

[azerupi]

This has been changed recently, anchors are now url encoded.

A section like ## Hello World should produce an anchor of the form page.html#Hello%20World. If that is not the case, it is a bug.

[dtolnay]

I would strongly prefer #hello-world. I don't like sharing or receiving links containing nasty urlencoded symbols because people want to know what they are clicking. GitHub agrees.

[azerupi]

I think I have to agree too.
It seems reasonable to change it to a more user-friendly form.