add support for javascript and typescript

[hatemhosny]

this PR adds support for files using IJavascript and ITypescript kernels.

as discussed in #249

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This change is 

[mwouts]

Excellent! Thanks @hatemhosny . I will review this shortly.

[codecov]

Codecov Report

Merging #250 into master will increase coverage by 0.06%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #250      +/-   ##
==========================================
+ Coverage   99.07%   99.14%   +0.06%     
==========================================
  Files          66       66              
  Lines        6286     6294       +8     
==========================================
+ Hits         6228     6240      +12     
+ Misses         58       54       -4

Impacted Files	Coverage Δ
jupytext/kernels.py	91.66% <ø> (ø)	⬆️
tests/test_pep8.py	98.36% <ø> (ø)	⬆️
jupytext/languages.py	100% <ø> (ø)	⬆️
jupytext/contentsmanager.py	98.43% <100%> (ø)	⬆️
jupytext/paired_paths.py	97.36% <100%> (ø)	⬆️
tests/test_mirror.py	100% <100%> (ø)	⬆️
jupytext/metadata_filter.py	100% <100%> (ø)	⬆️
jupytext/cell_reader.py	96.96% <0%> (+0.2%)	⬆️
jupytext/pep8.py	100% <0%> (+4.76%)	⬆️

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