Associate .scm file extension with Scheme scripts

[bzinberg]

Scheme support is already in place. This PR associates the .scm file extension with Scheme scripts, and extends the test to test that extension along with the existing .ss extension.

[codecov]

Codecov Report

Merging #192 into master will decrease coverage by 0.07%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #192      +/-   ##
==========================================
- Coverage   99.44%   99.36%   -0.08%     
==========================================
  Files          61       61              
  Lines        5537     5538       +1     
==========================================
- Hits         5506     5503       -3     
- Misses         31       35       +4

Impacted Files	Coverage Δ
jupytext/languages.py	100% <ø> (ø)	⬆️
tests/test_mirror.py	100% <100%> (ø)	⬆️
tests/test_read_simple_scheme.py	100% <100%> (ø)	⬆️
jupytext/pep8.py	95.23% <0%> (-4.77%)	⬇️
jupytext/cell_reader.py	98.75% <0%> (-0.25%)	⬇️

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[mwouts]

Thanks @bzinberg for your contribution. Definitely we should include this!

[mwouts]

By the way, I see there is an issue with the tests (pypy and Python 3.4, when jupytext is executed through subcommand). The issue is not related to your contribution, please ignore it.