add pyplot savefig()

[torlarse]

I am using Ripser.py alongside a Docker container running all the python stuff. This makes plotting tricky if at all possible. The workaround is simply to use pyplot.savefig() and save the plots as files.

This pull request is a naive and humble attempt to enable figure saving functionality to Ripser.py. Suggestions, edits and/ or corrections are welcome.

[codecov-io]

Codecov Report

Merging #25 into master will decrease coverage by 0.3%. The diff coverage is 0%.

@@            Coverage Diff             @@
##           master      #25      +/-   ##
==========================================
- Coverage   80.03%   79.73%   -0.31%     
==========================================
  Files          10       10              
  Lines         531      533       +2     
  Branches      105      106       +1     
==========================================
  Hits          425      425              
- Misses         78       79       +1     
- Partials       28       29       +1

Impacted Files	Coverage Δ
persim/visuals.py	57.94% <0%> (-1.11%)	:arrow_down:

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