Expressions

- Implemented Nibbler::getWord.
- Re-implemented Nibbler::getDOM.
- Modified DOM addressed for context-based attributes from "due" to ".due",
  the help disambiguate DOM references in expressions.  There is now a
  consistency:

      <id>.due         Task-specific
          .due         Contextual
    <uuid>.due         General

- Implemented associated unit tests.

src/Nibbler.h

@@ -66,6 +66,7 @@ public:
   bool getDate (const std::string&, time_t&);
   bool getOneOf (const std::vector <std::string>&, std::string&);
   bool getDOM (std::string&);
+  bool getWord (std::string&);
 
   bool skipN (const int quantity = 1);
   bool skip (char);