Add Erlang option for using maps instead of dicts

Erlang OTP 18 introduces significant performance improvements when using maps instead of dicts. Here is a benchmark showing the gains:

https://gist.github.com/BinaryMuse/bb9f2cbf692e6cfa4841

Thrift should support an option for the Erlang compiler to produce maps instead of dicts, to take advantage of these performance improvements.