A Marked 2 custom processor that renders reST files with rst2html and leaves the rest be.

gistfile1.py

#!/usr/bin/env python

from __future__ import print_function

import os
import sys
import subprocess

ext = os.environ['MARKED_EXT'].lower()

if ext == 'rst':
    print(subprocess.check_output(
        ['/usr/local/share/python/rst2html.py',
         '--no-xml-declaration', '--stylesheet='],
    ))
else:
    print('NOCUSTOM', file=sys.stderr)