Allow extended parsing to be disabled

[blakeembrey]

Expands upon the format API #129 with an extended option that can be used to disable the *= feature. I expect to add a couple more flags around parsing behavior to handle multipart and the other MIME encoding format so this is a safe place to introduce our first option. I don't expect it to be used much, but it may be useful for MIME or other contexts that support UTF-8 headers and therefore don't need the extended behavior.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (2bb7da3) to head (e51636c).

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #130   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines          223       236   +13     
  Branches        81        85    +4     
=========================================
+ Hits           223       236   +13     

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