Add markdown highlighting support in a md multiline string macro

[BenLauwens]

A small addition to julia.json to allow markdown highlighting support in a md multiline string macro. Successfully tested with visual-studio-code.

[pfitzseb]

We had these multiline macro strings defined earlier, but there are systematic issues with that: the injected grammar is not guaranteed to "terminate" (e.g. for unclosed code blocks in markdown, IIRC) at the closing triple quote.

ref https://github.com/JuliaEditorSupport/atom-language-julia/pull/252, https://github.com/JuliaEditorSupport/atom-language-julia/issues/244#issuecomment-962670836