Expose optional keyword for proto3

[mwatson2]

Hi @mafintosh - optional in proto3 has the meaning that the absence of the field can be distinguished from its default value. At the moment this module is swallowing all occurrences of the optional keyword so that users cannot see when fields have been marked this way.

I would suggest adding an optional boolean beside required and repeated. required would still always be false for proto3.

Would you accept a PR with this change ?

[mwatson2]

Here is a Pull Request: #69