Relative URLs in feeds are broken

[maximevaillancourt]

Relative links in feeds don't work, since they're expanded to the wrong domain. For instance,

/relative/url/example

is currently expanded to

https://json-feed-viewer.herokuapp.com/relative/url/example

when it should expand to

//feed.domain/relative/url/example

[paulradzkov]

I believe just adding <base href="//feed.domain"> to html head will be enough.