CLI: Export database as CSV

* Changed `Extract` to `Export` to support additional formats
* Allow database expot as CSV.  Added a `--format` option to the `Export` command for that, which defaults to xml, so the current behavior is unchanged.
*The `CsvExporter` had to be refactored a bit, but nothing major. It can
now print to a file or return a string.

tests/TestCli.h

@@ -52,7 +52,7 @@ private slots:
     void testEdit();
     void testEstimate_data();
     void testEstimate();
-    void testExtract();
+    void testExport();
     void testGenerate_data();
     void testGenerate();
     void testKeyFileOption();