Add opml2clj command

Most users migrating to feeds2imap.clj have an OPML file with their subscriptions. Writing `urls.clj` manually is a tedious task.

This PR adds a new command (`opml2clj`) to feeds2imap to convert an OPML file to its `urls.clj` format. Tested with some Google Reader and Feedly OPMLs. Only one level of `<outline>`'s nesting is supported (i.e. only a flat list of folders).