Fixed tooltipHoverFormatter not being deserialized as a function

[SparkyTD]

Fixes #449

I added 'tooltipHoverFormatter' to the list of JSON keys that will be evaluated as a function in the browser.

[SparkyTD]

I think there might be a more flexible way of solving this, using an attribute on the C# side, and some custom serialization/deserialization logic.

I'll look into the feasibility of that solution, and I might create another PR if it looks good.

[SparkyTD]

@joadan I have implemented the idea I was referring to: fucntion-serializer. It basically uses a custom JsonConverter to prefix all function strings with a custom JSON key ('@eval'), and replaces the name checking logic on the JS side with a much simpler logic that checks if a string is prefixed with this key.

So instead of serializing a function as myFunction: "function(arg) { ... }", it gets serialized as such:

myFunction: {
    "@eval": "function(arg) { ... }"
}

This way the client can simply check if the value is an object containing the '@eval' key, and use that to decide whether or not to evaluate the contents. It also works for function arrays that use CustomFunction.

Please tell me if this looks good, and I should open another PR.

[joadan]

@SparkyTD, this sounds great!
If you create a PR I'll take a look at it.

Thank you for your work!

[SparkyTD]

No problem, I love this project!

I created the PR: #453