Support for processing individual files/streams

[rufuspollock]

Want to be able to do something like this especially in cloudflare workers.

// stream is result of fs.open or similar
markdowndb.processMarkdown(stream) => the result of parsing a single file

We want this so that you can use markdowndb functions to process single rather than have to run on a whole directory.

That means we can process individual markdown files in e.g. cloudflare workers.