Use cross-env to pass env vars across platforms

[apostolos]

What: Fix test & build scripts in Windows

Why: rollup UMD builds need UMD=true to be set as an environment variable. This breaks test & build scripts on Windows machines making it hard for people to contribute.

How: Adds cross-env which takes care of differences to setting and using environment variables across platforms.

Checklist:

• Documentation N/A
• Tests N/A
• Code complete N/A

Only tested this in Windows and Linux. I don't currently have access to an OSX machine.

[codecov]

Codecov Report

Merging #327 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #327   +/-   ##
=======================================
  Coverage   97.33%   97.33%           
=======================================
  Files          17       17           
  Lines         562      562           
  Branches      133      133           
=======================================
  Hits          547      547           
  Misses         11       11           
  Partials        4        4

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[tkh44]

I can test this more tonight. Thank you for taking the time to help others @apostolos