Add support for non-ASCII URLs

[VladYermakov]

As feedbag uses URI.parse and Open::URI we've started getting tons of errors like the following:

URI::InvalidURIError: URI must be ascii only "https://www.rehabsociety.org.hk/zh-hant/\u4E3B\u9801/feed/"

Workaround here is to escape the URL before passing it to feedbag, but it'd be really useful if feedbag will be able to work with non-ASCII URIs (aka IRI)

Thanks in advance