Stateful binding handlers/constructors

[brianmhunt]

A common pair of problems across both internal and custom binding handlers seems to be:

1. Re-using information, which is generally handled with a closure in the bindingHandler init, as in textInput, value and others; and
2. Extending the functionality of core bindings, as in How to create one way (from view to datasource) textInput binding? #1601.

I have a relatively thorough comment on that issue, but the gist is to add allow Knockout to take bindingHandlers that are constructors.

I am likely missing something fairly important, but the key to this would seem to be in bindingAttributeSyntax

I am confident this could be done in a backward compatible way.

I will look at this more later, but would of course love thoughts on this.

[mbest]

Concerning the issues that have come up about extending binding handlers, I had a different idea: #663.