Feature - simple version report for scripts

- Added a simple _version command that displays only the version
  number and a newline.  This makes it easier for external task support
  scripts to determine which version of task is installed - easier than
  parsing the version command output.

src/main.h

@@ -68,6 +68,7 @@ int handleCompletionTags (std::string &);
 int handleCompletionCommands (std::string &);
 int handleCompletionIDs (std::string &);
 int handleCompletionConfig (std::string &);
+int handleCompletionVersion (std::string &);
 int handleVersion (std::string &);
 int handleDelete (std::string &);
 int handleStart (std::string &);