Funny highlight for header inside quote

[cben]

Users might not realize this is a continued quote:

> quote
# Header 0123456789

They'll have to learn. (One of these things where having a "markdown lint" would help)

But here things are made worse by 2 bugs, only Header is colored green as a quote.

• The # is bright-bluish - should fix CSS cascading.
• 0123456789 is blue, somehow due to GFM styling it as github SHA link and my kludge unstyling it.

[cben]

Moved to cben/mathdown#39 where it belongs