Add per-type override methods

[Tarmil]

This:

JsonFSharpOptions.Default()
    .WithOverrides(fun o -> dict [
        typeof<Foo>, o.WithOverrideMembers(dict [
            "Field1", [ JsonNameAttribute "MyField" ]
        ])
    ])

could be nicer like this:

JsonFSharpOptions.Default()
    .WithOverride<Foo>(_.WithOverrideMember("Field1", [ JsonNameAttribute "MyField" ]))

It would also give us an opportunity to give the options in the function passed to WithOverride a type parameter, which in turn could be used by WithOverrideMember:

JsonFSharpOptions.Default()
    // Typed field accessor, possible because we know the type being overridden!
    .WithOverride<Foo>(_.WithOverrideMember(_.Field1, [ JsonNameAttribute "MyField" ]))

Or even:

JsonFSharpOptions.Default()
    // The second type parameter is the type of Field1, it can be replaced with _
    .WithOverride<Foo, int>(_.Field1, [ JsonnameAttribute "MyField ])