Counter(): add 'one thousand' param. (#657)

* Counter(): add 'one thousand' param.

Needed for https://github.com/google/benchmark/pull/654

Custom user counters are quite custom. It is not guaranteed
that the user *always* expects for these to have 1k == 1000.
If the counter represents bytes/memory/etc, 1k should be 1024.

Some bikeshedding points:
1. Is this sufficient, or do we really want to go full on
   into custom types with names?
   I think just the '1000' is sufficient for now.
2. Should there be a helper benchmark::Counter::Counter{1000,1024}()
   static 'constructor' functions, since these two, by far,
   will be the most used?
3. In the future, we should be somehow encoding this info into JSON.

* Counter(): use std::pair<> to represent 'one thousand'

* Counter(): just use a new enum with two values 1000 vs 1024.

Simpler is better. If someone comes up with a real reason
to need something more advanced, it can be added later on.

* Counter: just store the 1000 or 1024 in the One_K values directly

* Counter: s/One_K/OneK/