Add support for decoding non-const enums.

[rhofour]

This required bumping up the target to ES2017. The syntax is a little redundant since you need the enum both as a parameter and a template parameter, but it doesn't look like there's a way to get the values of an enum at compile time from the template parameter.

[joanllenas]

Hi @rhofour, thanks for your contribution!
The code part looks perfect, just a couple of minor things.

• The target field in the tsconfig.ts file can stay as 'es5' and everything will work just fine. We need that in order to support ie11.
• Could you please add a documentation section in the readme file following the format of the rest of the file?

Thanks!

[joanllenas]

See #25 (comment)

[rhofour]

Done. I also bumped the version.

[joanllenas]

Thanks! the new version is ready!
https://github.com/joanllenas/ts.data.json/releases/tag/v1.3.0