Code Cleanup

- Factorize code for parsing date elements.
- Better order of blocks for parsing date elements.
- Add corresponding minimal-digit date parsing method for reading seconds,
  minutes and week.
- Update documentation and test.

src/Nibbler.h

@@ -78,6 +78,7 @@ public:
   bool getPartialUUID (std::string&);
   bool getDateISO (time_t&);
 #ifdef NIBBLER_FEATURE_DATE
+  bool parseDigits(std::string::size_type&, int&, unsigned int, bool strict = true);
   bool getDate (const std::string&, time_t&);
 #endif
   bool getOneOf (const std::vector <std::string>&, std::string&);