Support conditional compilation

还没有人认领这个 Issue。

On codeplex this was a popular feature request:

https://typescript.codeplex.com/workitem/111
https://typescript.codeplex.com/workitem/1926

Personally I think preprocessor directives like #if, #elif, #else #endif with possibility to specify symbol to compiler would be very useful.
And a way to mark function (or ambient function declaration) as Conditional (something like [ConditionalAattribute](http://msdn.microsoft.com/en-us/library/system.diagnostics.conditionalattribute%28v=vs.110%29.aspx) in C#) would be great improvement too.
I have lot's of `console.log` like function calls that decrease performance of my application and ability to easily remove all calls to this function would be great.

在定义范围之前，先查看链接的两个 CodePlex 功能请求以及此 issue 中的讨论。确定是否需要条件编译、编译器符号、条件函数，还是全部都需要，然后为受支持的指令和可移除的调用制定验收标准。

由索引模型根据 Issue 内容生成。