Expose callback function to element

[TimvdLippe]

This allows users to set custom callbacks to modify the outputted HTML. We use this in our project to modify GitHub issue references as follows:

<marked-element markdown="[[body]]" callback="[[callback(owner,name)]]"></marked-element>

callback: function(owner, name) {
  return function(err, text) {
    return text.replace(/&?#(\d+)/g, function(match, digits) {
      // Character was escaped, we should modify these numbers
      if (match.charAt(0) === '&') {
        return match;
      }
      return '<a href="https://www.github.com/' + owner + '/' + name + '/issues/' + digits + '" target="_blank">#' + digits + '</a>';
    });
  };
}

[TimvdLippe]

Friendly ping. Still relying on my own fork for this feature.

[notwaldorf]

/cc @FredKSchott

[TimvdLippe]

Friendly ping, this very small change allows us to mimic the GitHub markdown flavors.

[FredKSchott]

Oh man, thought I had looked at this already 😞
@TimvdLippe thanks for the reminder! I'll take a look at this later today.

[FredKSchott]

Do we want errors to be swallowed by default? I assume this changes marked's behavior to propagate errors asynchronously instead of throwing them synchronously. Can this be null/undefined instead?

[TimvdLippe]

Yes I have turned it into null per https://github.com/chjj/marked/blob/master/lib/marked.js#L1148. Sorry for the long wait, I was on vacation 😂

[FredKSchott]

no problem, for this PR I'd be a hypocrite to complain :)

[FredKSchott]

👍 New feature looks great! Just 1 question around default behavior.

[FredKSchott]

LGTM, Thanks @TimvdLippe! Merging now