Separate a part of building params

This change is useful for overriding the structure of standard params.

I use X-editable with Rails.
The structure of params for Rails is different from X-editable's default, so I write a library to override it.
https://github.com/tkawa/bootstrap-editable-rails/blob/master/vendor/assets/javascripts/bootstrap-editable-rails.js.coffee
But this code seems more than necessary because it rewrites params forcibly by hooking URL param.

If this change is applied, my library will just look like this:
```js
EditableForm.prototype.buildParams = function(value, pk) {
  // Build a structure suitable for Rails
};
```

It is very simple and readable.

Of course I can override params by giving function to option, but the option is for users of library.
I want this change for developers of library like me.