Make icon file's public path configurable

[jetpacmonkey]

Right now, the generated icon file is expected to be found inside the /static directory. My original rationale for using that instead of the publicPath in the webpack config was because <use> tags don't support cross-origin requests, and publicPath is often used to set to a CDN's domain. A better, more widely usable solution would be to take a config option in the plugin's constructor, have publicPath as a setting, and fallback to the webpack default publicPath if one is not specified.