adding myst init command

[choldgraf]

This adds a simple MyST python function as well as CLI command. It currently returns an error if there are multiple available kernels and no kernel manually specified, and we can make this an "interactive choice" in the future if we like. There are a few edge cases I couldn't figure out how to test for (e.g. how to deal w/ multiple kernels and single kernel environments) but they didn't seem like they were super critical to figure out.

[codecov]

Codecov Report

Merging #97 into master will decrease coverage by 0.26%.
The diff coverage is 89.79%.

@@            Coverage Diff             @@
##           master      #97      +/-   ##
==========================================
- Coverage   92.64%   92.38%   -0.27%     
==========================================
  Files          10       11       +1     
  Lines         517      565      +48     
==========================================
+ Hits          479      522      +43     
- Misses         38       43       +5     

Flag	Coverage Δ
#pytests	92.38% <89.79%> (-0.27%)	⬇️

Impacted Files	Coverage Δ
jupyter_book/utils.py	86.53% <75.00%> (-7.22%)	⬇️
jupyter_book/commands/__init__.py	100.00% <100.00%> (ø)
jupyter_book/tests/test_utils.py	100.00% <100.00%> (ø)

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