Add callback stops for univariate optimization.

[pkofod]

Fixes #822

[codecov]

Codecov Report

Merging #831 into master will increase coverage by 0.22%.
The diff coverage is 93.54%.

@@            Coverage Diff             @@
##           master     #831      +/-   ##
==========================================
+ Coverage   81.64%   81.86%   +0.22%     
==========================================
  Files          43       43              
  Lines        2446     2460      +14     
==========================================
+ Hits         1997     2014      +17     
+ Misses        449      446       -3     

Impacted Files	Coverage Δ
src/univariate/solvers/brent.jl	95.65% <87.50%> (+5.32%)	⬆️
src/univariate/solvers/golden_section.jl	97.61% <100.00%> (+0.47%)	⬆️

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