Support for negation in ConditionalAttribute

[maxkoshevoi]

As can be seen in this or this post it's a common problem.

If ConditionalAttribute is a better way to write #if around method calls, than it would be useful to support negation since#if supports it and it's commonly used, so that developers could effortlessly refactor their code to use ConditionalAttribute.

And there wouldn't be need to write something like this:

#if !DEBUG
[Conditional("FALSE")]
#endif

#if !DEBUG
#define NOT_DEBUG //define a symbol specifying a non debug environment
#endif

[Conditional("NOT_DEBUG")]