Option to parse basic markdown vs extended flavors

[2colours]

Hi,

this issue is inspired by converting README.md files of Raku modules (to pod6). Since GH itself uses markdown, I'm gonna show the raw text, its expected formatting (by simply adding it to my comment) and the actual formatting (experienced by converting the pod6 back to markdown):

Input:

#### new(Str $xml, :$filename)

Expected output:

new(Str $xml, :$filename)

Actual output:

new(Str xml, :filename)

By the way, this is similar to the formatting that show up in PR's on github: $'s have special meaning to those as well.

Overall, it would be great to be able to specify what sort of Markdown the input should be interpreted as, and generate the output based on that.