new: Fields and message expansion feature

[pamburus]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 98.30724% with 40 lines in your changes missing coverage. Please review.

Project coverage is 76.73%. Comparing base (4f0578e) to head (6263bdc).
Report is 14 commits behind head on master.

Files	Patch %	Lines
src/app.rs	94.62%	17 Missing ⚠️
src/cli.rs	93.20%	7 Missing ⚠️
src/formatting.rs	99.55%	6 Missing ⚠️
src/model.rs	80.95%	4 Missing ⚠️
crate/heapopt/src/lib.rs	97.61%	3 Missing ⚠️
crate/mline/src/lib.rs	99.28%	2 Missing ⚠️
src/fmtx.rs	90.90%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #290      +/-   ##
==========================================
+ Coverage   72.04%   76.73%   +4.68%     
==========================================
  Files          40       42       +2     
  Lines       10533    12484    +1951     
==========================================
+ Hits         7589     9580    +1991     
+ Misses       2944     2904      -40     

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