Add equality operators to AttrSpec

By adding `__eq__` and `__ne__` methods to `AttrSpec`, we can now check if two objects have the same colors or not.

`__hash__` is necessary to keep AttrSpec hashable.
Which other flags? The foreground and background attributes also have the
bold, underline, blink and standout flags. Is there anything else?

Anyway, it's cleaner and probably faster to just compare _value instead, so I
changed it.