[MRG] Make matplotlib optional

[vinayak-mehta]

No description provided.

[codecov-io]

Codecov Report

Merging #190 into master will decrease coverage by 0.68%.
The diff coverage is 81.31%.

@@            Coverage Diff            @@
##           master    #190      +/-   ##
=========================================
- Coverage   87.98%   87.3%   -0.69%     
=========================================
  Files          13      13              
  Lines        1248    1252       +4     
  Branches      298     297       -1     
=========================================
- Hits         1098    1093       -5     
- Misses        101     109       +8     
- Partials       49      50       +1

Impacted Files	Coverage Δ
camelot/cli.py	85.18% <54.54%> (-6.82%)	⬇️
camelot/plotting.py	89.85% <89.85%> (-1.93%)	⬇️

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