Reinstate 0-argument format functions

As discussed in issue #8, not having a 0-argument version of the format
functions is annoying and not compatible with printf, so this commit
reinstates it.

Unfortunately there doesn't seem to be a way to make the variadic
function simulation macros equivalent to the C++11 version, so the
0-argument versions are just written out in full for the C++98
implementation.

closes issue #8