Option to generate enum definition

Since https://github.com/Azure/autorest.typescript/issues/357 was merged it would be useful to optionally output the enum definitions as well as the union type.

e.g.

//current
type SomeEnum = 'one' | 'two' | 'three';

//proposed addition (with a better name)
enum SomeEnumOptions = {
  one = 'one',
  two = 'two',
  three = 'three'
}

Is this something that you would consider (pending some decisions about names etc)? If so I will look into what changes are needed.

[ghost]

https://github.com/Azure/autorest.typescript/pull/413 adds the ability to output enum types again by using the --enum-types command line argument.

[XiaoningLiu]

@daschult We need enum type for Azure Storage SDKs to avoid breaking changes, please bring back this.