This introduces the ability to run the migration offline, while
TL is not running as the migrations can take a long time.
The migrate command is added as a `thelounge storage` subcommand.
Reason being that it is expected that more subcommands will follow,
say `thelounge storage clean` to remove partial data from the db.
Make `thelounge install file:~/path/to/package` work rather than
erroring out that the folder doesn't exists.
Probably funny on Windows, but it doesn't hurt either