Add -m option to allow user specified marked options

Allow the user to override marked options for things like enabling github
flavored markdown with line breaks.
refs: https://github.com/chjj/marked#usage

Invoked via:
docco -m marked.json <filename.ext>

Sample marked.json file:
{
  "gfm": true,
  "breaks": true,
  "smartypants": true
}