Properly merge custom and default renderers

[Exilz]

There was a problem with the custom renderers prop. If you provide some custom renderers, (ie : only for <h2>), the default one will not be merged with your custom ones.

Before I realized it, <img> weren't showing anymore on my app.

I merged HTMLRenderers and the optionnal renderers prop in the constructor so I could use it on line 90.

I left untouched defaultProps.

[aronnebrivio]

+1 please merge, this solution works