Enhancement - export.yaml

- Preliminary export.yaml support.  No unit tests yet, and no decision
  on including this feature.  It may be that libyaml is the right choice,
  as an optional dependency.

src/main.h

@@ -133,6 +133,7 @@ int handleImport (std::string&);
 // export.cpp
 int handleExportCSV (std::string &);
 int handleExportiCal (std::string &);
+int handleExportYAML (std::string &);
 
 // list template
 ///////////////////////////////////////////////////////////////////////////////