API for plugins to collaboratively render a node

[hukkin]

The current API lets plugins add new renderer functions (for node types that dont have a default one) or override existing functions. There is no good way for plugins to collaborate on the same node type, however.

This would be useful e.g. for plugins to set character escape rules by collaboratively modifying renderers of paragraph and text syntaxes. This would solve issues like executablebooks/mdformat-tables#10

[hukkin]

Closed by #175