viewportUnitExpression is matching base64 strings

[badsyntax]

See: https://github.com/rodneyrehm/viewport-units-buggyfill/blob/0c5f0776f23141ae57b78f863dec87179c3a5781/viewport-units-buggyfill.js#L28

This expression need to be stricter to prevent matching characters within a base64 encoded string. This issue was identified in our application, where this buggyfill script was "corrupting" base64 encoded images.

I wonder if simply adding word boundary metacharacters around the expression will suffice, but as this repository does not contain any sort of automated testing tools, I cannot be sure, and I don't really have the time to manually test this change.

Here's the change: https://gist.github.com/badsyntax/bc4423dc51b04f54ae52/revisions