WIP: Visual regression tests

[gustaphe]

An idea to solve #78 , using VisualRegressionTests.jl
Not ready or even sure if it's that useful, but I wanted to see if I could get CI to collaborate.

Also needed an option to render without transparency, which should probably be implemented either way.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 82.89%. Comparing base (4a3e41b) to head (3d2d8ca).
Report is 9 commits behind head on master.

Files	Patch %	Lines
src/utils.jl	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #303      +/-   ##
==========================================
+ Coverage   82.87%   82.89%   +0.02%     
==========================================
  Files          21       22       +1     
  Lines         905      912       +7     
==========================================
+ Hits          750      756       +6     
- Misses        155      156       +1     

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[gustaphe]

There we go. We could probably come back to this before we make any big changes (like #161)