vdl: add the ability to specify go struct tags via the vdl.config file.

[cosnicolaou]

This PR adds support for specifying go struct tags via the vdl.config file. The example below will add the tag json:"ja,omitempty" to TestStruct.A.

config = vdltool.Config{
        Go: {
                StructTags: map[string][]vdltool.GoStructTag{
                        "TestStruct": []vdltool.GoStructTag{
                                {Field: "A", Tag: `json:"ja,omitempty"`},
                        }
                },
        },
}