Support passing in markdown-it `env`

[segevfiner]

markdown-it can receive an env object in its parse call that can be used to retrieve and pass in some information into the parser for some plugins and token types. It would be nice to be able to pass it in.

[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.