Add matlab in a language and script list

[shhong]

This adds the matlab language and extension '.m' in a language and script list. It works surprisingly well!

[codecov]

Codecov Report

Merging #197 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #197      +/-   ##
==========================================
+ Coverage   99.46%   99.46%   +<.01%     
==========================================
  Files          62       62              
  Lines        5565     5571       +6     
==========================================
+ Hits         5535     5541       +6     
  Misses         30       30

Impacted Files	Coverage Δ
jupytext/languages.py	100% <ø> (ø)	⬆️
tests/test_mirror.py	100% <100%> (ø)	⬆️
jupytext/magics.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 2fb5d7a...dba755a. Read the comment docs.

[mwouts]

Thanks @shhong . That's great! I was not aware that Jupyter notebooks could also contain Matlab code...

May I ask you to also contribute a sample matlab notebook in tests/notebook/ipynb_m ? And, if possible, a Python notebook with a few matlab cells in tests/notebooks/ipynb_py ? You can also commit the text files generated by the tests, or if you prefer I'll do that later on.

Also, you may also want to mention Matlab as a supported language in the README.md (e.g. add Matlab every time you find Julia).

[mwouts]

@shhong, your contribution has been integrated in Jupytext 1.0.3. Thanks !