Render matplotlib figures (similar to sphinx gallery)

[sizmailov]

Generated images stored in global output directory under matplotlib-figures sub-directory.

TODO:

• tune html/css to make it look pretty
• name output images similarly to source files
• add customization options for finer render control (css/html-wise) ?

Depends on #145

test page render


[codecov[bot]]

Codecov Report

Merging #161 into master will increase coverage by 1.74%. The diff coverage is n/a.

@@             Coverage Diff             @@
##           master      #161      +/-   ##
===========================================
+ Coverage   98.25%   100.00%   +1.74%     
===========================================
  Files          27         1      -26     
  Lines        6635       353    -6282     
  Branches       44        44              
===========================================
- Hits         6519       353    -6166     
+ Misses        116         0     -116     

Impacted Files	Coverage Δ
documentation/doxygen.py
plugins/m/htmlsanity.py
plugins/m/abbr.py
plugins/m/dox.py
plugins/m/metadata.py
plugins/dot2svg.py
plugins/m/code.py
plugins/m/sphinx.py
plugins/m/components.py
plugins/m/math.py
... and 16 more

---

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[sizmailov]

Finally this PR starts looking like something (check out spoiler in first comment)! :fireworks: :tada:

Two (minor) remaining problems pointed out in first comment still exist and second one is worth discussion.

[mosra]

Similarly as with #145 I have reconsidered my position on this PR -- since this is a rather niche feature and the functionality is quite standalone, I think it could live on its own outside of the m.css repository, maintained by a third party instead. M.css itself will only contain the m.plots plugin with functionality reduced to a subset where I can have full control over how the output looks like.

I hope that's understandable.