store the precompile statements from the precompile execution scripts

[KristofferC]

Shows the file getting executed and the resulting statement file:

[ Info: PackageCompiler: Executing /home/kc/JuliaPkgs/PackageCompiler.jl/precompile_example.jl => /tmp/jl_packagecompiler_gtWkhz/jl_cZBXfb
Hello, bro!
[ Info: PackageCompiler: Done

Closes #336

[codecov]

Codecov Report

Merging #571 (5e749b9) into master (e9f7075) will decrease coverage by 0.78%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #571      +/-   ##
==========================================
- Coverage   84.28%   83.50%   -0.79%     
==========================================
  Files           2        2              
  Lines         490      491       +1     
==========================================
- Hits          413      410       -3     
- Misses         77       81       +4     

Impacted Files	Coverage Δ
src/PackageCompiler.jl	82.30% <100.00%> (-0.85%)	⬇️

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