Add valueOrF to EitherT

[denisrosca]

In certain circumstances the default value might be wrapped in the F[_] context already. I think valueOrF belongs in EitherT.

[codecov-io]

Codecov Report

Merging #2192 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2192      +/-   ##
==========================================
+ Coverage   94.73%   94.73%   +<.01%     
==========================================
  Files         330      330              
  Lines        5582     5585       +3     
  Branches      209      209              
==========================================
+ Hits         5288     5291       +3     
  Misses        294      294

Impacted Files	Coverage Δ
core/src/main/scala/cats/data/EitherT.scala	97.65% <100%> (+0.05%)	⬆️

---

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 e3a12ef...e8a0d85. Read the comment docs.

[ceedubs]

I like this, and it mirrors nicely with getOrElseF.