Add orientation argument to allow flipping axes

[krassowski]

Closes #135

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 61.38614% with 39 lines in your changes missing coverage. Please review.

Project coverage is 76.72%. Comparing base (f93a687) to head (ad8d763).
Report is 34 commits behind head on master.

Files	Patch %	Lines
R/upset.R	61.38%	39 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #142      +/-   ##
==========================================
- Coverage   77.88%   76.72%   -1.17%     
==========================================
  Files           4        4              
  Lines        1542     1594      +52     
==========================================
+ Hits         1201     1223      +22     
- Misses        341      371      +30     

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