Parse url login info into basic auth header

Fixes #124 (closed). Since hyper doesn't do anything with username:password in urls, we need to convert it to an Authorization header manually.

This will probably become unnecessary if/when !88 (closed) is merged, but I think this is worth having in the meantime. A good chunk of the podcasts I listen to are behind a basic auth wall, so this feature is vital for my use.

I don't know Rust very well, so apologies if this isn't idiomatic!