Provide an array of available date input formats

This will allow to fix the current hardcoding of strings describing
date input formats in multiple places.

Signed-off-by: Baptiste Jonglez <baptiste--git@jonglez.org>
Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

src/calcurse.h

@@ -940,6 +940,7 @@ extern unsigned colorize;
 extern int foreground, background;
 extern enum ui_mode ui_mode;
 extern int read_only;
+extern const char *datefmt_str[DATE_FORMATS];
 extern int days[12];
 extern const char *monthnames[12];
 extern const char *daynames[8];