Add support for starred label

[stve]

This PR adds support for starring messages that match a filter.

Thanks so much for writing this library! I just consolidated so many of my filter rules into a nice tidy config file 😄

[codecov-io]

Codecov Report

Merging #18 into master will increase coverage by 0.63%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #18      +/-   ##
==========================================
+ Coverage   15.41%   16.04%   +0.63%     
==========================================
  Files           4        4              
  Lines         266      268       +2     
==========================================
+ Hits           41       43       +2     
  Misses        221      221              
  Partials        4        4              

Impacted Files	Coverage Δ
filter.go	27.33% <100.00%> (+1.06%)	:arrow_up:

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