Move string array concatenation to format.h.

Move string array concatenation to format.h.

- Moved code from Options::setDescription() that concatenated strings
  from an array into format.h to make it more generally usable.
- Changed the code such that instead of requiring the array to be
  NULL-terminated, it can also deduce the length of the array passed as
  an argument.
- Made Options::setDescription() to take a simple string to allow using
  the alternative overload, and also to help in writing test code
  (for #666).

Change-Id: I96f45577fc6a4258a1c993983cc1d1a1a0846bdc