Add two new functions string_catftime(), resp. string_strftime(), which
can be used to append, resp. print, a formatted date to a dynamic
string.
Signed-off-by: Lukas Fleischer <lfleischer@calcurse.org>
Introduce support for strings which support concatenating the current
value with a printf-style formatted value, thereby growing in size
dynamically.
Signed-off-by: Lukas Fleischer <lfleischer@calcurse.org>
