add cache_path option to wily.cfg

[skarzi]

Closes: #117

[codecov-commenter]

Codecov Report

Merging #123 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #123   +/-   ##
=======================================
  Coverage   97.98%   97.99%           
=======================================
  Files          23       23           
  Lines        1143     1145    +2     
=======================================
+ Hits         1120     1122    +2     
  Misses         23       23           

Impacted Files	Coverage Δ
src/wily/config.py	100.00% <100.00%> (ø)

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[skarzi]

We can also discuss config loading and processing more deeply, so it can be refactored a bit to make adding new options easier and safer, but also produce more meaningful messages for invalid option's values

[tonybaloney]

Brilliant! thanks for the contribution. Sorry I missed this earlier