add possibility to transform/map binded values about plates

[tauren]

+1 This is similar to what I suggested in #29

[kkolman]

yep, what you're suggesting is more advanced and flexible

just adding that my branch uses the "with" method which is a reservd keyword also (as is class) in JS/ECMAscript

[3rd-Eden]

Closing this, you can now do:

var map = new plates.Map();
map.className('pewpew').use(function render(data, value, tagbody) {
 return 'pewpew' + data.pewpew + value;
});