Support for language-markdown scopes

[avonmoll]

This syntax theme works great for GFM scopes, i.e. source.gfm, markup.heading.heading-3.gfm, etc. However, it looks like duotone syntaxes don't catch Markdown scopes associated with language-markdown, i.e. text.md, heading.markup.md, etc.

with language-markdown activated:

with language-pfm activated: