Consider adding a hook for plugins to disable other plugins

[hukkin]

executablebooks/mdformat-myst#9 (comment)

[LecrisUT]

I think best way to design this is to read for .mdformat.yaml in each folder, and merge the options of the last given .mdformat.yaml. Then expose an option of a dict where we define the keys as the plugins we want to load, and the values are how we configure, e.g.:

#.mdformat.yaml
plugins:
  mdformat-gfm:
---
#docs/.mdformat.yaml
plugins:
  mdformat-myst:
    args: [ --dollarmath ]
    exclude: README.md
  mdformat-gfm:
    include: README.md