Spec validation customization

[p1c2u]

Allows to disable spec validation that can improve the performance. Relates to #289

[codecov]

Codecov Report

Merging #290 (accd9cf) into master (3719092) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #290      +/-   ##
==========================================
- Coverage   96.75%   96.74%   -0.01%     
==========================================
  Files         122      122              
  Lines        2431     2429       -2     
==========================================
- Hits         2352     2350       -2     
  Misses         79       79              

Impacted Files	Coverage Δ
openapi_core/schema/shortcuts.py	100.00% <100.00%> (ø)
openapi_core/schema/specs/factories.py	100.00% <100.00%> (ø)

---

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 3719092...accd9cf. Read the comment docs.