Add two codegen parameters to nano.

enum_style = c | java: 'c' to put the enum member int constants
at the parent scope; 'java' to create uninstantiatable shell
classes at the parent scope and put the int constants inside.

optional_field_style = default | accessors: 'default' to create
one public mutable field per optional proto field; 'accessors'
to encapsulate the generated fields behind get, set, has and
clear accessors.

This CL only contains parsing code for these two parameters.

Change-Id: Iec0c3b0f30af8eb7db328e790664306bc90be089