Add support for external diff commands

[allenporter]

Add support for running an external diff command, and update README with examples with dyff.

Issue #101

[codecov-commenter]

Codecov Report

Merging #108 (a7c067d) into main (d8f87f1) will decrease coverage by 0.73%.
The diff coverage is 86.48%.

@@            Coverage Diff             @@
##             main     #108      +/-   ##
==========================================
- Coverage   92.13%   91.41%   -0.73%     
==========================================
  Files          15       15              
  Lines        1399     1432      +33     
==========================================
+ Hits         1289     1309      +20     
- Misses        110      123      +13     

Impacted Files	Coverage Δ
flux_local/tool/diff.py	84.57% <84.61%> (-6.28%)	⬇️
flux_local/command.py	96.82% <88.88%> (-1.33%)	⬇️
flux_local/tool/selector.py	98.24% <100.00%> (+0.06%)	⬆️
flux_local/git_repo.py	95.63% <0.00%> (+0.39%)	⬆️

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