add setters with DateTime

[andy128k]

Closes #106

New methods have suffix _strict. I am not super excited with this and am open to suggestions.

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Codecov Report

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@@            Coverage Diff            @@
##             master     #108   +/-   ##
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  Coverage          ?   86.19%           
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  Files             ?       25           
  Lines             ?     1927           
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  Hits              ?     1661           
  Misses            ?      266           
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Impacted Files	Coverage Δ
src/channel.rs	88.03% <100.00%> (ø)
src/item.rs	92.90% <100.00%> (ø)

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[kitlith]

I don't have any immediate ideas on a better name, though it would be nice if we could do it with the builders, too.

Frankly, I think most of my difficulties were because I started with a chrono::NaiveDate. It may be sufficient to just note in documentation that "you can easily format a date as rfc822 with chrono::DateTime::to_rfc2822()" and leave it at that.

[andy128k]

@kitlith

Examples how to use this crate with chrono are added in #112.