Filter nested metadata

[mwouts]

Filtering nested cell and notebook metadata is now possible! This closes #416.

[codecov]

Codecov Report

Merging #425 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #425      +/-   ##
==========================================
+ Coverage   98.93%   98.94%   +<.01%     
==========================================
  Files          75       75              
  Lines        7608     7645      +37     
==========================================
+ Hits         7527     7564      +37     
  Misses         81       81

Impacted Files	Coverage Δ
jupytext/cell_to_text.py	97.3% <100%> (ø)	⬆️
jupytext/formats.py	98.66% <100%> (ø)	⬆️
tests/test_metadata_filter.py	100% <100%> (ø)	⬆️
jupytext/version.py	100% <100%> (ø)	⬆️
jupytext/compare.py	99.33% <100%> (-0.02%)	⬇️
jupytext/metadata_filter.py	100% <100%> (ø)	⬆️
jupytext/jupytext.py	98.59% <100%> (ø)	⬆️
tests/test_contentsmanager.py	100% <100%> (ø)	⬆️
jupytext/combine.py	100% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 3a6512d...d4500fc. Read the comment docs.