asciidoc: add support for extra filters

Current list of extra filters:
 - ditaa
 - mscgen
 - blockdiag (includes seqdiag, actdiag, nwdiag and packetdiag)

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 filters, put this in packageOverrides:

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