allow custom comparison function

[Raynos]

The current comparison just does equality on the arguments.

Maybe we want a custom comparison that knows how to handle arguments that are "tuples" i.e. shallow comparison on array arguments.

[Matt-Esch]

Custom comparison function now possible in #9:

var createPartial = require('vdom-thunk/partial');

module.exports = createPartial(function (currentArgs, previousArgs) {
    // currentArgs and previousArgs are an array of args passed into
    // the thunk for the current and previous thunks
    return false;
});