Re-engineer istanbul code coverage dependencies

[mattcollier]

The instanbul code coverage libraries used here are not being maintained properly. In particular, there is a JS parser module used to instrument code that is now a number of major releases behind. There are some bugs in the version of the library being used that results in valid JS code not being parsed properly. This will result in webpack failing to build the browser bundle. In cases where this is a problem, I have created this branch that disables the istanbul code coverage.

d00f7ce

Here's a link to the unmaintained library that needs to be replaced somehow:

webpack-contrib/istanbul-instrumenter-loader#96

[aljones15]

any idea when this will hit a release? This caused issues for me today.