Is it possible to write a custom converter for Pandoc?

Is it possible to write a custom output writer for Pandoc?

For example, suppose I want to convert a document:

pandoc -f markdown -t myCustomMarkup asdf.md

Does Pandoc have a way I can specify the conversion rules for myCustomMarkup? (e.g. I could specify that text that had the 'bold' attribute should map to <bold>text</bold>, and so on for all features/attributes that Pandoc recognises).

Can anyone point me to some documentation as to how I can implement my own? I can't seem to find any mention of this.

(Additionally, is there a way to "plug in" a writer defined in a file without having to (say) re-compile pandoc? e.g. pandoc -f markdown -t myCustomMarkup --writerpath=path/to/my/writer asdf.md)