Add ability to generate int enums

[holykol]

This PR adds @intEnum directive, which, when applied, changes enums to be generated with int values.
Sample generated code: https://gist.github.com/holykol/c8056f4bb80c0b17e3478a85cbd27be2

I have:

• Added tests covering the bug / feature (see testing)
  Updated any relevant documentation Any suggestions for where to put documentation for this new feature?

closes #366

[coveralls]

Coverage decreased (-0.2%) to 66.055% when pulling 2a8c8ce on holykol:int-enums into 3a31a75 on 99designs:master.

[tarrencev]

would love this!

[mpieczaba]

Looking forward to the merge...

[kateile]

Any progress on this?

[flymedllva]

I think what's missing is a directive to set the digits for the value by itself. For the same proto, we can set numbers randomly. This would be useful along with using the goModel directive on proto types

As an example

directive @intEnum on ENUM

type TestType @goModel(model: "package.Type") {
    testStatus: TestStatus!
}

enum TestStatus @intEnum {
    NULL # 0
    CREATE @intEnum(value: 50) # 50
    MODIFY # 1
    DELETE @intEnum(value: 100) # 100
}