Take a regex to identify the icon file in webpackIsomorphicParser

[jetpacmonkey]

Currently, webpackIsomorphicParser has a hard-coded const iconsFileRE = /icons-[a-f0-9]+\.svg/i;. Ideally, there'd be some way for the parser to access the plugin from the webpack config, and get the filename from there. A slightly lesser solution would be to simple take in the same pattern that the plugin does, and let the user deal with keeping that in sync.