Add configureYulOptimizer option

[cgewecke]

#636, #248

Variant of #642 ... making this optional for v0.7.x.

Would like to experiment with a few other versions before defaulting to this config.

[codecov-commenter]

Codecov Report

Merging #659 (b1d7201) into master (ba94880) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #659   +/-   ##
=======================================
  Coverage   98.68%   98.68%           
=======================================
  Files          22       22           
  Lines         987      991    +4     
=======================================
+ Hits          974      978    +4     
  Misses         13       13           

Impacted Files	Coverage Δ
plugins/hardhat.plugin.js	93.47% <100.00%> (+0.29%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ba94880...b1d7201. Read the comment docs.