breaking: Remove FeedHandler class

[fb55]

To migrate, change

const handler = new FeedHandler((err) =>
    cb(err, handler.feed)
);

to

const handler = new DomHandler((err) =>
    cb(err, getFeed(handler.dom))
);

[coveralls]

Pull Request Test Coverage Report for Build 2211907891

• 20 of 20 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 99.222%

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Totals
Change from base Build 2207460022:	0.2%
Covered Lines:	1914
Relevant Lines:	1929

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💛 - Coveralls