v0.2.0-alpha: recursive definitions

Full Changelog: v0.1.0-alpha...v0.2.0-alpha

changes

• Recursive keys are now supported, with the feature of catching circular references. Comprehensive coverage of this is covered in the manual
• New commands: LIST, RESOLVE
• Moved error messages to a header file, removed static const std::runtime_error and opted for macro strings instead
• Bugs associated with multidimensional lists were fixed in the parser
• Program exits are handled through main.cpp when prompted by handler.cpp