Allow user to set name of campaign output

[joezuntz]

See CSL issue 115

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (88d3e01) 59.88% compared to head (98be95c) 59.87%.

Files	Patch %	Lines
cosmosis/campaign.py	87.50%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #111      +/-   ##
==========================================
- Coverage   59.88%   59.87%   -0.01%     
==========================================
  Files         115      115              
  Lines       11846    11849       +3     
==========================================
+ Hits         7094     7095       +1     
- Misses       4752     4754       +2     

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