Add support for custom Enum values in schema

[creeefs]

Hello,

Per the discussion here graphql/graphql-js#525, I was wondering if there were any plans to implement custom Enum values.

[creeefs]

From scanning the issues list, it looks like there was already a discussion to add support for custom enum values in https://github.com/apollographql/core-docs/issues/178. That issue has since been closed. @helfer If you could provide some guidance on the implementation, I could take a stab and open a PR.

[hronro]

Any updates?

[orta]

This was fixed in #508