asciidoc: support extra filters (ditaa, mscgen, diag)

Also, slightly change the shebang fixup in the patchPhase so that it
handles optional [[:space:]] before the interpreter path (needed for the
filters).

To enable the the extra filters, put this in packageOverrides:

  asciidoc = pkgs.asciidoc.override {
    enableDitaaFilter = true;
    enableMscgenFilter = true;
    enableDiagFilter = true;
  };