Add enumerator values in documentation

[jnikula]

The Sphinx C and C++ domain directive for enumerators allows optionally specifying the enumerator value:

.. c:enumerator:: name = value

It would be straightforward to unconditionally add it (the Cursor for CursorKind.ENUM_CONSTANT_DECL has enum_value property), but the question is whether that is the right thing to do.

The alternatives:

• Always display value
• Display value if the source code explicitly sets it. This is what Doxygen appears to do. I think this requires looking at the tokens. I've implemented this in Enumerator values #176
• Make it configurable, could be done globally and at c:autoenum::
• Never display value (status quo)

I'm not really fond of adding a lot of configuration, and would prefer sane defaults instead.

@BrunoMSantos thoughts?

[BrunoMSantos]

I replied in #176, but to be explicit, I like the Doxygen behaviour here. I don't see much value in making it configurable personally and we can always add that later if anyone wants it I guess, keeping the Doxygen behaviour as default.