Default label to last merged PR

[hipstersmoothie]

What Changed

When running auto label the pr number is no longer required. It will now find the last merged PR and return it's labels

Why

This enables auto label to be run in master after a PR has been merged. You can use the labels to automate more things in your build pipeline other than the release.

to enable #22

Todo:

• Add tests
• Add docs
• Add SemVer label

[codecov]

Codecov Report

Merging #83 into master will increase coverage by 0.1%.
The diff coverage is 64.7%.

@@            Coverage Diff            @@
##           master      #83     +/-   ##
=========================================
+ Coverage   75.13%   75.23%   +0.1%     
=========================================
  Files          13       13             
  Lines         925      941     +16     
  Branches      128      135      +7     
=========================================
+ Hits          695      708     +13     
- Misses        202      206      +4     
+ Partials       28       27      -1

Impacted Files	Coverage Δ
src/cli/args.ts	92.68% <ø> (+2.43%)	⬆️
src/github-release.ts	95.16% <100%> (+0.07%)	⬆️
src/git.ts	99.37% <100%> (+0.02%)	⬆️
src/main.ts	23.78% <14.28%> (-0.22%)	⬇️

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