add gem about variadiac functions

[wilzbach]

From @wilzbach on May 17, 2016 20:30

http://dlang.org/variadic-function-templates.html

Copied from original issue: stonemaster/dlang-tour#126

[wilzbach]

From @stonemaster on May 18, 2016 7:17

I am stil undecided on this one. Maybe it could be too little value added because using the basic knowledge one gets quickly acquainted to variadic functions (at least those in D).

[wilzbach]

Maybe we can add it as part of another gem?