Split install and configure commands

[jayqi]

Splitting install command

• Split install into two commands:
  • install installs the post-save hook initialization in the Jupyter config
  • configure creates the .nbautoexport configuration file in a directory
• When running the configure command, if the post-save hook initialization is absent or out-of-date, there will be a warning printed.
• Add a new --jupyter-config option to install if user wants to install the post-save hook initialization to a Jupyter config in a nonstandard location.

Other

• Found a bug from #30 where new clean configuration option was not being passed through install_sentinel. Fixed by changing the interface of install_sentinel to accept a config object instead specifying each option. This reduces likelihood of similar future bugs by reducing the number of places that reproduce the NbAutoexportConfig interface.
• Replace pkg_resources.parse_version with packaging.version.parse. This is what pkg_resources calls under the hood and is a lighter-weight dependency.
• Downside of really high test coverage is then it becomes a pain to strictly maintain. Add a .codecov configuration to:
  • Allow up to a 2% decrease in coverage against base
  • Remove the reach graph, which I never found that interpretable but is really visually obnoxious.

Closes #34. I did not unify the naming and opened #37 separately to avoid adding noise to this PR's diff.

[codecov[bot]]

Codecov Report

Merging #38 into master will decrease coverage by 0.1%. The diff coverage is 96.9%.

@@           Coverage Diff            @@
##           master     #38     +/-   ##
========================================
- Coverage    98.3%   98.1%   -0.2%     
========================================
  Files           8       8             
  Lines         357     381     +24     
========================================
+ Hits          351     374     +23     
- Misses          6       7      +1     

Impacted Files	Coverage Δ
nbautoexport/nbautoexport.py	98.2% <96.0%> (-0.7%)	:arrow_down:
nbautoexport/jupyter_config.py	96.5% <100.0%> (+0.1%)	:arrow_up:
nbautoexport/sentinel.py	100.0% <100.0%> (ø)

[jayqi]

@r-b-g-b I think the issue you had should be fixed. Ready for another look.