Add --label-selector command line flags

[allenporter]

Fixes #832

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 87.50000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 93.39%. Comparing base (17a72a8) to head (270195c).

Files with missing lines	Patch %	Lines
flux_local/tool/selector.py	87.50%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #839      +/-   ##
==========================================
- Coverage   93.43%   93.39%   -0.04%     
==========================================
  Files          21       21              
  Lines        2377     2393      +16     
==========================================
+ Hits         2221     2235      +14     
- Misses        156      158       +2     

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