Dict validation: Keep track of matched input keys

[sjakobi]

Fixes #80.

[codecov-io]

Current coverage is 99.37%

Merging #92 into master will increase coverage by +0.01% as of 0988074

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Review entire Coverage Diff as of 0988074

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[guilload]

Maybe you could use 2 sets directly instead of the matched dict.

[sjakobi]

Yeah, of course. Thanks! :)