Consider adding is option to EnumOptions

[yuri-sergiichuk]

It's currently possible to add (is).java_type option to a particular message, but not to an enum which could be useful as well.

E.g. consider the following example with enums used for CI build states:

    // The build state.
    enum State {
        S_UNKNOWN = 0;

        // The build is created.
        CREATED = 1;

        // The build is received by the CI.
        RECEIVED = 2;

        // The build is in progress.
        STARTED = 3;

        // The build has passed successfully.
        PASSED = 4;

        // The build has failed.
        //
        // Denotes that the developer code could not be successfully built.
        //
        FAILED = 5;

        // The build configuration has failed.
        //
        // Denotes that the build configuration or pre-build steps failed.
        //
        ERRORED = 6;

        // The build is cancelled.
        CANCELLED = 7;
    }

We may want to have a way to augment the enum with smth like isFailed() where FAILED, ERRORED and CANCELLED statutes are all considered as failures.

[yuri-sergiichuk]

Closing in favor of a more generic #69.