Expose default handlers and an API to provide custom ones

[coreyjv]

I recently wrote my own code to convert from ADF to an MDAST-like structure. I discovered this utility and I was eager to use it. However, I discovered that this utility doesn't expose an API to provide custom handlers similar to mdast-util-to-markdown.

A capability like this would let me implement custom transformations for existing elements that I need to behave differently or implement transformations for ones that are "skipped" in this implementation.

Thanks!

[andreasknoepfle]

Hey @coreyjv. We are happily accepting contributions. Since we are mainly using this lib ourselves in https://github.com/bitcrowd/tickety-tick I doubt that we will have any time & effort spent on this any time soon (could be but I can't promise anything), since there it is not really a usecase for us to have this in tickety-tick. But if you want to get started on it we would most likely support you in adding such an API for custom transformations.