Allow optional marshalling JSON without quotes

[olablt]

Is it possible to add an option to marshal/unmarshal without quotes?

like shopspring/decimal@d6f5224

[nikki-dzhurov]

@eapenkin Can you please take a look into the PR ? I have a wrapper around the type to achieve the same effect. It would be great to be supported by the library :)

[LFS-dtschan]

@eapenkin Would be great if the supplied PR could be merged