Support for unexported structs

[FZambia]

I continue my experiments with easyjson and results look really impressive at moment.

The problem I came across while adopting easyjson to Centrifugo is that it does not support code generation for unexported structs - i.e. I get lots of errors like cannot refer to unexported name libcentrifugo.apiCommand when run easyjson -all commands.go for this file. I can make all structs exported in the end of course but hope there is a simple way to support them too by easyjson.