enh add HorizontalRule to markdown

[hayd]

Adds horizontal rule, <hr />, like this one:

---

cc @one-more-minute (Note: this precedence required for this appears to be before paragraph.)

[hayd]

Hmmm, weird/suprising behaviour of the @md_str macro is the reason for the failing test:

julia> md"Hello\n\n---\nWorld"
  Hello\n\n–-\nWorld

julia> md"""Hello

       ---

       World"""
  Hello

  ---------------------------------------------------------------------------------------------------------------------------

  World

julia> Markdown.parse("Hello\n\n---\nWorld")
  Hello

  ---------------------------------------------------------------------------------------------------------------------------

  World

Pushed a fix.

[MikeInnes]

Seems like the build failure is just a timeout, I think that can be ignored.

Is the md"" behaviour that surprising? I don't think \n style escaping is part of CommonMark – to me it would be more surprising if typing paths and such entered random newlines.

[hayd]

Fair enough, that that makes sense. I expected "a\nb" to always behave the same as an actual new line (as it does in julia), I do still think it is a little surprising that it doesn't...

Thanks for looking/merging all these!

[MikeInnes]

Thank you for the contributions!