[Enhancement] Support OPML as the RSS feeds configuration

[librehat]

Just found this project by chance.

I may have some free time myself and want to see whether you'll be open to such enhancement. OPML is commonly used by many RSS readers as a way to exchange RSS feeds. When using OPML, we can build all the RSS feeds in an OPML file as a single book. Potentially we can expand the current conf/release.js to allow the feeds specified as a path to an OPML.

[gonejack]

That will be great, your PR is welcome.

My pain about this project is epub-gen didn't accept this PR cyrilis/epub-gen#72 which will make lot of errors when this project doing long time running.