Support passing in markdown-it `env` about prosemirror-markdown

[marijnh]

What kind of thing is this used for? Would you want to pass it in to the parse method, or the MarkdownParser constructor?

[segevfiner]

To the parse method.

See https://markdown-it.github.io/markdown-it/#MarkdownIt.parse & https://github.com/markdown-it/markdown-it/blob/master/docs/architecture.md#data-flow

[marijnh]

I know what that argument is, what I was asking is what you are using it for.

[segevfiner]

To pass and receive environmental data from some custom markdown-it plugin.

[marijnh]

Added in attached patch.