add support for filtering by label selectors

[ryane]

fixes #9

[codecov-io]

Codecov Report

Merging #10 into master will increase coverage by 8.77%.
The diff coverage is 90.32%.

@@            Coverage Diff             @@
##           master      #10      +/-   ##
==========================================
+ Coverage      78%   86.77%   +8.77%     
==========================================
  Files           6        6              
  Lines         150      189      +39     
==========================================
+ Hits          117      164      +47     
+ Misses         24       17       -7     
+ Partials        9        8       -1

Impacted Files	Coverage Δ
pkg/filter/errors.go	85.71% <ø> (ø)	⬆️
pkg/filter/filter.go	100% <100%> (+22.85%)	⬆️
pkg/filter/matcher.go	84.21% <85%> (+8.7%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update cea9ebc...031077b. Read the comment docs.

[Stanislasss]

Thanks @ryane for the fast reaction. I'll have a look at the new feature :-)