Added --bt-save-metadata option. When true is given, it saves
metadata as .torrent file. This option has effect only when
BitTorrent Magnet URI is used. The filename is name in metadata
with suffix .torrent. The directory to be saved is the same
directory where download file is saved. If the same file already
exists, metdata is not saved.
* src/OptionHandlerFactory.cc
* src/UTMetadataPostDownloadHandler.cc
* src/UTMetadataPostDownloadHandler.h
* src/message.h
* src/prefs.cc
* src/prefs.h
* src/usage_text.h
* test/UTMetadataDataExtensionMessageTest.cc