Add grep -v command

[ontanj]

Add grep -v command and some tests. Also fix a bug that gives overlapping matches for grep -C. Fix issue yetibot/yetibot#980 in yetibot repo.

[codecov[bot]]

Codecov Report

Merging #151 into master will increase coverage by 0.01%. The diff coverage is 40.00%.

@@            Coverage Diff             @@
##           master     #151      +/-   ##
==========================================
+ Coverage   41.91%   41.93%   +0.01%     
==========================================
  Files          94       94              
  Lines        4070     4073       +3     
  Branches      190      190              
==========================================
+ Hits         1706     1708       +2     
- Misses       2174     2175       +1     
  Partials      190      190              

Impacted Files	Coverage Δ
src/yetibot/core/commands/collections.clj	27.19% <40.00%> (+0.36%)	:arrow_up:

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[devth]

Thanks @ontanj !