Support subscribing to feeds with NewsBlur

[Screwtapello]

Currently, Awesome RSS allows me to pick "RSS" to use Firefox's native RSS support (removed in Firefox 64), or a variety of other services I don't use or recognise.

Apparently you can make NewsBlur subscribe to a feed by substituting the feed's URL for %s in the following:

http://www.newsblur.com/?url=%s

As an alternative, perhaps you could just add a preference for an arbitrary URL, which I think is #117.

[shgysk8zer0]

@Screwtapello I'd prefer arbitrary URLs. I've had too many service support requests to implement them all without overwhelming users and creating a terrible options page.

Some more popular ones (and I don't have any criteria for determining that cutoff) will be added, especially if they seem more popular with typical users (unlike the ones that might indicate that the user can figure out how to add custom feed URLs).