add setting to disable enum conversion

[zbyte64]

Needs unit test but not entirely sure about patching settings.

Resolves #67

[coveralls]

Coverage increased (+0.005%) to 94.523% when pulling 4f58783 on zbyte64:add_enum_choice into f76f38e on graphql-python:master.

[firaskafri]

@zbyte64 whats the alternative to enum to guarantee correct queries/mutations?

[zbyte64]

Way I see it there are 3 options:

• Create one converter to rule them all. This is what we're doing right now, I don't think it will ever work.
• Option to disable enum globally. More like a stopgap solution, should be avoided
• Option to explicitly set the enum value. This is probably the right way to go. Check out Support formatting of enum choices #209