limit keyword split to first match, allow colons to be part of the keyword

[fwuehr1995]

Possible solution for https://github.com/doorstop-dev/doorstop/issues/631

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (31dcabc) 96.25% compared to head (8ee57fd) 96.25%.

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Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #634   +/-   ##
========================================
  Coverage    96.25%   96.25%           
========================================
  Files           41       41           
  Lines         5160     5160           
  Branches      1214     1214           
========================================
  Hits          4967     4967           
  Misses         121      121           
  Partials        72       72           

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