Error when upgrading to v5.0.0
After upgrading to v5.0.0 I'm getting an error (see below).
This happens because the `publicPath` fallback [has been removed](https://github.com/jeerbl/webfonts-loader/pull/71/files#diff-168726dbe96b3ce427e7fedce31bb0bcL169).

In my Webpack config I'm not setting the [`publicPath` ](https://webpack.js.org/guides/public-path/) option. This option is now inherited by webfont-loader, which means `undefined` in my case and thus can't be resolved.

Created a PR to fix this #75

```
ERROR in ./assets/js/icons.font.js (./node_modules/css-loader/dist/cjs.js!./node_modules/webfonts-loader!./node_modules/babel-loader/lib!./assets/js/icons.font.js)
Module build failed (from ./node_modules/webfonts-loader/index.js):
TypeError [ERR_INVALID_ARG_TYPE]: The "url" argument must be of type string. Received type undefined
    at Url.parse (url.js:152:11)
    at urlParse (url.js:146:13)
    at Object.urlResolve [as resolve] (url.js:657:10)
    at webfontsGenerator (/root/defacto-elements/node_modules/webfonts-loader/index.js:223:28)
    at /root/defacto-elements/node_modules/@vusion/webfonts-generator/src/index.js:106:4
    at _fulfilled (/root/defacto-elements/node_modules/q/q.js:854:54)
    at /root/defacto-elements/node_modules/q/q.js:883:30
    at Promise.promise.promiseDispatch (/root/defacto-elements/node_modules/q/q.js:816:13)
    at /root/defacto-elements/node_modules/q/q.js:624:44
    at runSingle (/root/defacto-elements/node_modules/q/q.js:137:13)
    at flush (/root/defacto-elements/node_modules/q/q.js:125:13)
    at process._tickCallback (internal/process/next_tick.js:61:11)
```