Support enums in .fbs

[vaind]

Example FBS that should be supported, with appropriate enum generated in c/c++ code:

enum Flags: uint(bit_flags) {
    FlagA = 0,
    FlagB = 1,
}

table Table {
    id: ulong;
    flags: Flags;
}

[greenrobot]

Something to consider (especially for non bit flags): in C++ most people prefer enum classes.