[WIP] Add a shouldUpdate option to useTracker

[CaptainN]

Change the third parameter (currently undocumented) to accept an option object with two optional nodes - shouldUpdate and the already present computationHandler. This proposed change would make it possible to reduce rerenders by providing a comparison function, to check old values, with new values.

• [ ] Test assumptions, and do any kind of testing
• [ ] Adjust the computationHandler tests to use the option
• [ ] Add tests for the shouldUpdate option
• [ ] Add to Readme documentation

@menelike

[CaptainN]

I'd actually like to make this an optional 2nd OR 3rd argument. The second argument is a deps array, but I'd like to be able to accept a config object instead (overload style). This might make sense to do after the Typescript conversion lands, since there's now a split in the two versions of the hook - with/without deps, and the overload decision can be cleaning implemented there.