Add option to render diagrams after generation

[bkryza]

Fixes #204

[codecov-commenter]

Codecov Report

Merging #209 (146c79a) into master (429e1c4) will decrease coverage by 0.29%.
The diff coverage is 28.15%.

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@@            Coverage Diff             @@
##           master     #209      +/-   ##
==========================================
- Coverage   87.03%   86.74%   -0.29%     
==========================================
  Files         119      119              
  Lines       12950    13030      +80     
==========================================
+ Hits        11271    11303      +32     
- Misses       1679     1727      +48     

Files	Coverage Δ
src/common/generators/generators.h	96.07% <ø> (ø)
src/common/generators/plantuml/generator.h	87.39% <100.00%> (ø)
src/common/model/diagram_filter.cc	90.08% <100.00%> (ø)
src/common/types.h	89.55% <ø> (ø)
src/config/config.h	100.00% <ø> (ø)
src/config/option.h	100.00% <ø> (+2.77%)	⬆️
...m/generators/mermaid/sequence_diagram_generator.cc	95.86% <100.00%> (ø)
src/util/util.h	97.87% <ø> (ø)
src/config/yaml_decoders.cc	87.54% <90.90%> (+0.02%)	⬆️
src/config/config.cc	83.70% <60.00%> (-0.54%)	⬇️
... and 4 more

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