wip: add more default themes, use bootswatch

[aviatesk]

the objective of this PR is to add prepared themes and provider nicer way to tweak styles in general.

more details to come in soon

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• [x] provide helper API for syntax highlight theme
• [x] add build steps to download bootswatch themes
• [ ] add themes
• [ ] maybe use SASS.jl
• [ ] automatic theme matching
• [ ] documentation

[codecov-commenter]

Codecov Report

Merging #338 into master will decrease coverage by 0.43%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #338      +/-   ##
==========================================
- Coverage   74.23%   73.80%   -0.44%     
==========================================
  Files          25       25              
  Lines        1347     1355       +8     
==========================================
  Hits         1000     1000              
- Misses        347      355       +8     

Flag	Coverage Δ
#unittests	73.80% <0.00%> (-0.44%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/Weave.jl	50.00% <0.00%> (-0.58%)	:arrow_down:
src/plots.jl	0.00% <ø> (ø)
src/rendering/common.jl	81.52% <0.00%> (-6.72%)	:arrow_down:

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