Allows to include an external file which can set the markdown settings

Instead of providing the "marked" markdown settings through the apidoc binary i extended it with the ability to load an external file which allows to set the marked settings + extensions you want to the markdown parsing.

By doing:

```
apidoc --marked-config markdown_config.js
```

and have a file "markdown_config.js":

```
// Be including marked here we could overwrite the renderer really easy
var marked = require('apidoc/node_modules/marked');
var myRenderer = new marked.Renderer();

// A lot of the markdown rendering function can be altered this way
// eg: below we add target blank for each external link
myRenderer.link = function(href, title, text) {
  var external, newWindow, out;
  external = /^https?:\/\/.+$/.test(href);
  newWindow = external || title === 'newWindow';
  out = "<a href=\"" + href + "\"";
  if (newWindow) {
    out += ' target="_blank"';
  }
  if (title && title !== 'newWindow') {
    out += " title=\"" + title + "\"";
  }
  return out += ">" + text + "</a>";
};

// options can be set as wanted
module.exports = {
  gfm: true,
  tables: true,
  breaks: false,
  pedantic: false,
  sanitize: false,
  smartLists: false,
  silent: false,
  highlight: null,
  langPrefix: '',
  smartypants: false,
  headerPrefix: '',
  renderer: myRenderer,
  xhtml: false
};
```