fix node checking so we work on browsers and node

[cpirich]

• We were failing to load in the browser since webpack started wrapping this code and passing a local variable called global - fixed using the technique recommended here: How to conditionally include packages on client but not server (and vice-versa)? vercel/next.js#219

[codecov-io]

Codecov Report

Merging #47 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #47   +/-   ##
=======================================
  Coverage   34.62%   34.62%           
=======================================
  Files           4        4           
  Lines         543      543           
=======================================
  Hits          188      188           
  Misses        355      355

Impacted Files	Coverage Δ
src/loadP5.js	57.14% <100%> (ø)	⬆️

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[epeach]

I'm not super familiar with webpack and the cause of this error, but LGTM.