Custom renderers

[Yomguithereal]

Hello @rexxars. Sorry for this PR a bit heavy but I tried to follow up on the ideas of #5 and updated your code to be able to handle custom renderers for every types.

A rendering function has therefore the following signature:

function renderParagraph(node, props, children, opts) {

  // node is the commonmark parser node
  // props are the renderer's pre-computed props such as keys etc.
  // children are the component's children
  // opts are the renderer's options

  // the function must simply return a react element
  return <p {...props}>{children}</p>;
}

You can then pass your custom renderers likewise:

var renderer = new ReactRenderer({
  renderers: {
    Strong: function(node, props, children) {
      return React.createElement('b', props, children);
    },
    Text: function() {
      return 'Placeholder';
    },
    Paragraph: function(node, props, children) {
      return React.createElement('section', props, children);
    }
  }
});

// will render "This is some *important* text."
// into "<section>Placeholder<em>Placeholder</em>Placeholder</section>"

This has some collateral benefits: one can now very easily highlight code (without the lib imposing a precise method on him). I will probably, for instance, develop a code renderer using highlight.js on a project when have with @glenjamin.

Plus, node type solving is now O(1), compared to the O(n) switch statement and while the number of function calls has increased a bit, this should somewhat increase the renderer's performance.

What still remains to be discussed with this solution however is what should be passed to the allowNode function.

Here it is :). If you like it and find it suitable, I can also edit the PR to add some docs about custom rendering.

[Yomguithereal]

I also added a little fix concerning keys and warnings again.

[unstubbable]

+1

[rexxars]

You've done some tremendous work here, and I hate to let that work go to waste. However, I discovered a few bugs last week that I had to fix, and it ended up changing major parts of the code. In doing so, I realized that the road to custom renderers was pretty short, so I decided to take a stab at it.

We came up with something very similar, API-wise. I decided to just have the renderers take a params object, and also tried to explicitly define the properties that I wanted to pass on to the individual types. I hope you find this to be an acceptable solution.

I've co-credited you in the changelog since you took the time to do all this work, and I hope the new solution will be suitable for your needs. I'm hoping to release a new version tomorrow or on Sunday, along with a new version of react-markdown.

[Yomguithereal]

Thanks @rexxars. Happy to see we are going the same way :). I'll check the changes next week to see if I can help some more.