Dynamically load necessary files for the CodeMirror mode set through the mode option

Mode can either be a mime-type `text/html` or a CodeMirror mode-name `htmlmixed`
cf. http://codemirror.net/demo/loadmode.html

In this way the component is easier to set-up, but this approach might be suboptimal
if only one mode is required.

Thanks to Ian Johnson for debugging https://groups.google.com/d/msg/derbyjs/s4dMb1VA_DI/Q6uklwiIUfEJ