chore: add npm downloads per month

[nfischer]

Adds a gitter badge for # of downloads per month. We could switch this to number of total downloads if you think that would be more interesting.

[codecov-io]

Current coverage is 88.46% (diff: 100%)

Merging #79 into master will not change coverage

@@             master        #79   diff @@
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  Methods           0          0          
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  Hits             23         23          
  Misses            3          3          
  Partials          0          0          

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[ariporad]

LGTM!