Fix new text notebook

[mwouts]

• The notebook extension is activated by default
• new_untitled works with the default extensions ("" and ".ipynb")

Closes #443

[codecov]

Codecov Report

Merging #526 into master will decrease coverage by 0.00%.
The diff coverage is 95.45%.

@@            Coverage Diff             @@
##           master     #526      +/-   ##
==========================================
- Coverage   98.89%   98.88%   -0.01%     
==========================================
  Files          89       89              
  Lines        8628     8645      +17     
==========================================
+ Hits         8533     8549      +16     
- Misses         95       96       +1     

Impacted Files	Coverage Δ
jupytext/contentsmanager.py	96.84% <94.44%> (+0.22%)	⬆️
jupytext/version.py	100.00% <100.00%> (ø)
tests/test_contentsmanager.py	100.00% <100.00%> (ø)
jupytext/cell_metadata.py	98.32% <0.00%> (-0.34%)	⬇️

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