Import

- Added unit tests for the 'import' command, which uses JSON as the
  native format.  This is the new default import/export format.  All
  other import/export formats are (to be) implemented as transforms
  to JSON.

src/en-US.h

@@ -309,6 +309,7 @@
 #define STRING_CMD_DENO_FOUND        "Found annotation '{1}' and deleted it."
 #define STRING_CMD_DENO_NOMATCH      "Did not find any matching annotation to be deleted for '{1}'."
 #define STRING_CMD_IMPORT_USAGE      "Imports JSON files."
+#define STRING_CMD_IMPORT_SUMMARY    "Imported {1} tasks."
 
 // Config
 #define STRING_CONFIG_OVERNEST       "Configuration file nested to more than 10 levels deep - this has to be a mistake."