Add settings entries for --keep & --archive-dest

This allows a user to turn on `--keep` every time they run the program,
and to set the default archive destination when `--keep` is turned on.
Using `--archive-dest` overrides the archive destination in setting.ini.

aqt/settings.ini

@@ -5,6 +5,8 @@ concurrency: 4
 baseurl: https://download.qt.io
 7zcmd: 7z
 print_stacktrace_on_error: False
+always_keep_archives: False
+archive_download_location: .
 
 [requests]
 connection_timeout: 3.5