Add Clojure support

[bzinberg]

Adds support for Clojure notebooks.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@dda117f). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #193   +/-   ##
=========================================
  Coverage          ?   99.36%           
=========================================
  Files             ?       62           
  Lines             ?     5554           
  Branches          ?        0           
=========================================
  Hits              ?     5519           
  Misses            ?       35           
  Partials          ?        0

Impacted Files	Coverage Δ
jupytext/languages.py	100% <ø> (ø)
tests/test_read_simple_clojure.py	100% <100%> (ø)
tests/test_mirror.py	100% <100%> (ø)

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[mwouts]

Thanks @bzinberg for your contribution. I have included this into version 1.0.2.