enum values binding v2

[nikitacrit]

Hello @StevenACoffman @Desuuuu! This is continuation of original pr #2982.
Changes:

• Directive @goMode on enum values replaced with new directive @goEnum
• Added restrictions for invalid binding settings and indicating errors for the user
• Added variables support
• Added inpackage binding support
• Added recipe in docs

[coveralls]

coverage: 75.0% (-0.1%) from 75.124%
when pulling 211b49e on nikitacrit:enum-values-binding-v2
into ecc3f64 on 99designs:master.

[nikitacrit]

@StevenACoffman @Desuuuu ping 😊

[StevenACoffman]

I'm going to merge this and cut a release, but I consider @goEnum experimental, and I am open to breaking changes to it if the feedback from the community deems it so.

For instance, if an upcoming Go version adds native sum types (or real Enums that aren't iota) then I would expect this to change in a backwards incompatible way.