Support for using processors

[wooorm]

Take the following example:

var prose = unified()
  .use(retextParse)
  .use(equality)
  .use(quotes)

var html = unified()
  .use(minify)
  .use(rehypeStringify)

var markdown = unified()
  .use(remarkParse)
  .use(remark2retext, prose)
  .use(lint, {rules: {/*...*/}})
  .use(remark2rehype)
  .use(html)

Currently, the last line doesn’t work, we have to replace it with:

  .use(minify)
  .use(rehypeStringify)

...which is okay for two lines but when using many plugins it becomes
cumbersome, and hampers reusability.

If passing a processor to use would be supported we walk into a new
problem though: the given processor, if it has a parser or compiler,
would overwrite the origin parser/compiler with them. That’s OK for
the compiler (I think?) but not for the parser.