Added make clean command

[AddisonG]

Also added a .PHONY label.

There are some build systems which expect a clean command to exist, and many humans also do :)

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Sorry for the Pull Request spam. No need to make a release or anything for this.

Thanks for your work on this package.

[codecov[bot]]

Codecov Report

Merging #8 (3613af0) into master (b5f29e7) will increase coverage by 0.13%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master       #8      +/-   ##
==========================================
+ Coverage   66.25%   66.39%   +0.13%     
==========================================
  Files           1        1              
  Lines         243      244       +1     
==========================================
+ Hits          161      162       +1     
  Misses         82       82              

Impacted Files	Coverage Δ
lib/optparse/init.lua	66.39% <0.00%> (+0.13%)	:arrow_up:

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