issue-77: added logic to turn headers into atoms, covered it with tes…

[vad2der]

…ts, expanded documentation

This PR addresses Headers atomization issue 77

Open questions

• Anything that you wanted to discuss

Checklist

• Tests added Yes, for running only relevant tests: mix test --only atomize_headers
• Coverage increases or stays the same Increases
• Docs updated Yes, in CSV.Decoding.Decoder module and in README.md files.
• Changelog updated No, since changelog to be updated when release happens

[coveralls]

Coverage remained the same at 100.0% when pulling 40ee6fb5df1e6922141ada7fb8e7b2547e14caf2 on vad2der:issue-77 into ff96571e54cc57fb9ff8523a3c203614c114f4f7 on beatrichartz:master.

[beatrichartz]

See comment in #77 , closing this.