Add binding ability

Hi,

I've added the ability to create custom bindings, this is crucial for extendability, for example with these small additions, you can do 2-way bindings on one line:

https://github.com/jsguy/mithril.bindings

Also, see example code:

https://github.com/jsguy/mithril.bindings/blob/master/example.htm

*Note:* This does not change the default behaviour of Mithril, it only adds the ability to customise how bindings are handled.

As soon as these changes are accepted, I'll add the bindings project to the wiki - I really think this is a useful addition - let me know what you think!