Import/Export #35

There should be a quick way for users to import from and export to OPML (or other formats). This would require both logic and a user interface

Import from/export to OPML should be available from the command line, perhaps with these signatures:

arsse.php import <username> [<filename>] [-f | --flat] [-r | --replace]
arsse.php export <username> [<filename>] [-f | --flat]

In both cases, category attributes would be used to represent tags, and <outline> structure to represent folders, allowing both styles of organization to be used.

The --flat option, for export, would export the feeds without a folder structure, as this can sometimes be a compatibility issue. For import it would ignore any structure in the OPML file.

The --replace option would assume the OPML file is the desired final state of a user's feed structure, rather than a set of additions. This may involve moving folders or feeds, deleteing folders or feeds, removing tags, etc.