Extension methods for fluid-style chart styling and creation API.

Helper type for handling the special way the Plotly.NET core API uses generics. In short, the problem arises because many optional parameters of Plotly.NET's core API are generics with a type constraint for `IConvertible`. This means that these parameters can be both value and reference types (e.g. `double` and `System.DateTime` both implement IConvertible). If we now have a optional parameter of type `T? where T: IConvertible` the compiler will not allow this without further type constrainst to eithe reference or value type. This is a problem because we want to 1. allow both, and 2. have a reliable way of determining if the value was not set because the F# API expects to be passed `Option.None` in that case. There exist other workarounds like checking if the value is default or null, but that changes valid default values actually set to null as well.

Extension methods for the `Optional` class