Add a --no-skip-missing-interpreters option

[asottile]

Resolves #631

[asottile]

I wasn't sure whether to use versionadded or versionchanged, shrugs!

[obestwalter]

I dunno if this is really necessary.

[codecov]

Codecov Report

Merging #633 into master will increase coverage by 0.08%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #633      +/-   ##
==========================================
+ Coverage   93.86%   93.95%   +0.08%     
==========================================
  Files          11       11              
  Lines        2364     2365       +1     
==========================================
+ Hits         2219     2222       +3     
+ Misses        145      143       -2

Impacted Files	Coverage Δ
tox/config.py	97.58% <100%> (ø)	⬆️
tox/_verlib.py	83.15% <0%> (+2.1%)	⬆️

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[obestwalter]

see #634 for dicussion.

[asottile]

Thought you closed this, I'll do it!