Fix for IgnoreAltBlank when alt=""

[wjdp]

Previously would only ignore alts if missing, now does what it says it does and disables all alt checks.

Fixes #115

[codecov]

Codecov Report

Merging #120 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #120   +/-   ##
=======================================
  Coverage   86.53%   86.53%           
=======================================
  Files          20       20           
  Lines        1062     1062           
=======================================
  Hits          919      919           
  Misses        136      136           
  Partials        7        7

Impacted Files	Coverage Δ
htmltest/check-img.go	100% <100%> (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 4b5831f...662fb56. Read the comment docs.