new exercise diffie-hellman

Addresses https://github.com/exercism/xgo/issues/78.

It's an exercise in using big numbers.  For a language like Go that has big numbers, a random big number generator, and a modular exponentiation function, the solution doesn't require much code.  Still, I thought this exercise breaks down logically into steps.  The Go test program actually has an extra step (KeyPair) but I thought that step might seem redundant in object oriented languages so I left it out of the readme.  Also in the readme I used ** to indicate exponentiation.  Alternatives I considered were ^ and `<sup>` but I thought ^ might be slightly less well understood and `<sup>` wouldn't render in all contexts.

The Wikipedia URL in the .yml has a non-ASCII dash in it.  You might check that the escaping is okay the way it is.