ENH: provide formatting version of Foam::name() (issue #253)

- there are some cases in which the C-style sprintf is much more
  convenient, albeit problematic for buffer overwrites.

  Provide a formatting version of Foam::name() for language
  primitives that is buffer-safe.

  Returns a Foam::word, so that further output will be unquoted, but
  without any checking that the characters are indeed entirely valid
  word characters.

  Example use,
      i = 1234;
      s = Foam::name("%08d", i);
      produces '00001234'

  Alternative using string streams:

      std::ostringstream buf;
      buf.fill('0');
      buf << setw(8) << i;
      s = buf.str();

  Note that the format specification can also be slightly more complex:

     Foam::name("output%08d.vtk", i);
     Foam::name("timing=%.2fs", time);

It remains the caller's responsibility to ensure that the format mask
is valid.