Ignore JSON key that contains math operator

[kimlimjustin]

I noticed that when I generate the types by pasting JSON that contains "+" on the key, it will automatically pass it into the result, however, "+" is invalid in typescript.

Hope that this can be fixed by ignoring it.

Thanks

[RobertjanTuit]

Proposed fix: #26
You can point your package to https://github.com/RobertjanTuit/MakeTypes temporarily to use until is merged