Allow compressing to stdout like gzip and pigz on the isal.igzip CLI.

[rhpvorderman]

Checklist

• Pull request details were added to CHANGELOG.rst

This is necessary for implementing pycompression/xopen#48.

I added pytest tests for the CLI. The unittests copied from CPython worked well, but the python -m gzip behavior has some quirks that I removed from the python -m isal.igzip implementation. Also the unittests did not properly report the coverage.
I find that the pytest tests for the CLI interface are much cleaner as well.

[codecov]

Codecov Report

Merging #38 (d1f440f) into develop (49d1a5c) will increase coverage by 6.77%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop      #38      +/-   ##
===========================================
+ Coverage    88.97%   95.74%   +6.77%     
===========================================
  Files            2        2              
  Lines          136      141       +5     
===========================================
+ Hits           121      135      +14     
+ Misses          15        6       -9     

Impacted Files	Coverage Δ
src/isal/igzip.py	95.65% <100.00%> (+6.93%)	⬆️

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