Embedded purescript in markdown

[nwolverson]

Not strictly an issue for this repo but it's maybe not entirely obvious without looking into it that's the case.

To embed PureScript syntax highlighting in (eg) markdown fenced code blocks, the source.purescript scope has to be embedded in the grammar.

For language-gfm, look here: https://github.com/atom/language-gfm/blob/v0.90.5/grammars/gfm.cson#L287

For language-markdown, here: https://github.com/burodepeper/language-markdown/blob/master/grammars/fixtures/fenced-code.cson#L71

markdown-preview already supports PureScript, by virtue of generic mapping foo to source.foo