Handle multiple feeds in `fetch_feed()`.

[peterwilsoncc]

Handles passing an array to fetch_feed() by requesting each feed individually and then calling SimplePie\SimplePie::merge_items(); on the result before passing the data back in to a SimplePie object.

I don't love this so would love some suggestions of anything I am missing.

Trac ticket: https://core.trac.wordpress.org/ticket/64136

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