Add mode to automatically determine a count cutoff

[johnlees]

Closes #29

[codecov]

Codecov Report

Patch coverage: 79.62% and project coverage change: -1.80 ⚠️

Comparison is base (91d1a61) 82.69% compared to head (899e5fd) 80.90%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #34      +/-   ##
==========================================
- Coverage   82.69%   80.90%   -1.80%     
==========================================
  Files          15       16       +1     
  Lines        1647     1922     +275     
  Branches      288      355      +67     
==========================================
+ Hits         1362     1555     +193     
- Misses        104      142      +38     
- Partials      181      225      +44     

Impacted Files	Coverage Δ
src/cli.rs	67.37% <60.00%> (-0.57%)	⬇️
src/coverage.rs	79.78% <79.78%> (ø)
src/lib.rs	67.89% <88.23%> (-5.05%)	⬇️
src/ska_dict.rs	82.64% <100.00%> (ø)

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