Option to add future-proof "redundant" Other(value) in enum?

[jfrolich]

See: mhallin/graphql_ppx#69

GraphQL considers adding an enum to be a non-breaking change, because most type systems it seems aren't sound and so the developer always has to include a catch-all. In this world, adding a value to an enum means the old application should (mostly?) keep working, even when it encounters a value it doesn't understand.

Right now, we can't really do that for clients with graphql_ppx. It might be nice to have some option (or even required) to have a catch-all `Other(stringValue) to make apps much safer in the face of GraphQL's "non-breaking" [😄] changes.

[baransu]

Duplication of #5