Add support of bulk redefinition of token renderers and token renderer prototypes in the \markdownSetup command using enumeration

[Witiko]

Continues #232.

Compared to the syntax described in #232 (comment), I settled on the following syntax:

\markdownSetup {
  renderers = {
    heading(One|Two|Three) = {...},
  }
}

This is more versatile than the original design and does not conflict with TeX braces.