Make the markdown lib dependencies as a default and editable option

This adds 2 additional options, that give the possibility to use any markdown library we want.

E.g.:
```js
$("#target-editor").markdown({
  htmlToMarkdown: function(html) {
    return toMarkdown(html);
  },
  markdownToHtml: function(md) {
    return markdown.toHTML(md);
  }
})
```

By default these 2 options still use `markdown-js` or `marked` (so it's backward compatible with previous editor versions).

It should also fix https://github.com/toopay/bootstrap-markdown/issues/9.