Added more feed types

[JanPetterMG]

Added more feed types.

Pretty common:

<link rel="alternate" type="application/xml" href="https://example.com/feed/" />
<link rel="alternate" type="application/rdf+xml" href="https://example.com/feed/rdf/" />

Less common, but still used on a decent number of webpages:

<link rel="alternate" type="text/xml" href="https://example.com/feed/" />

[JanPetterMG]

These Embed\Exceptions\InvalidUrlException: Invalid url (...) issues in the Travis check that failed, is that a recently introduced issue?
I'm receiving lots of InvalidUrlExceptions for no obvious reason, using the latest v3.0.5.
I've just upgraded from v2.7.7 btw...

EDIT: Probably unrelated to issue 200, as the urls named in the exceptions doesn't include UTF-8 characters.

[oscarotero]

thanks

PD: The issues in the travis test is because changes in the webpages, not in this library. I've fixed them.