Add support for map<enum_type, some_other_type>

[c2h5oh]

Right now only basic types are supported

[pkieltyka]

@c2h5oh perhaps the issue is using enum_type as the key. webrpc might right now require string or number as the key, but any value is supported already as you can see in https://github.com/webrpc/webrpc/blob/master/_examples/golang-basics/example.ridl#L46

for enum_type, since its number based, should be pretty easy to add support for it eventually

[c2h5oh]

That's exactly what this ticket is about - being able to end up with map[enumType]valueType on go side

[pkieltyka]

@xiam this would be really nice to add support for too, and should be easy to support in parser since enum's are all numbers under the hood

[pkieltyka]

btw, im pretty sure once #55 is solved, then this ticket will be closed along with it