Add OPML importer for existing podcasts

[tofumatt]

Related to #28, it would be handy if we could import old podcasts from another app, presumably using OPML.

[lmorchard]

For bonus points: Not many services do this, but OPML can work as a kind of meta-subscription. That is, periodically poll an OPML list at an URL, and you can pick up changes in the list to add/remove podcast subscriptions managed outside the app.

For instance, a few people were managing lists of political casts during the 2008 elections. You could subscribe to that curated list, and then dump the whole thing when the election was over

You can also easily store arbitrary attributes on OPML list nodes, so it works as a kind of sync format too

[tofumatt]

I don't see this as an important feature for now. We can revisit it later after we have better search and sync support I guess. I'll close it for now.