Add ability to pass additional parameters to svgicons2svgfont
I have the following SVG...
``` svg
<?xml version="1.0" encoding="UTF-8"?>
<svg width="24px" height="24px" viewBox="0 0 24 24" version="1.1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
    <title></title>
    <defs></defs>
    <g id="chevron" stroke="none" stroke-width="1" fill="none" fill-rule="evenodd">
        <path d="M10.1464466,9.35355339 C9.95118446,9.15829124 9.95118446,8.84170876 10.1464466,8.64644661 C10.3417088,8.45118446 10.6582912,8.45118446 10.8535534,8.64644661 L13.8535534,11.6464466 C14.0488155,11.8417088 14.0488155,12.1582912 13.8535534,12.3535534 L10.8535534,15.3535534 C10.6582912,15.5488155 10.3417088,15.5488155 10.1464466,15.3535534 C9.95118446,15.1582912 9.95118446,14.8417088 10.1464466,14.6464466 L12.7928932,12 L10.1464466,9.35355339 Z" id="Mask" fill="#072456" fill-rule="nonzero"></path>
    </g>
</svg>
```

Which results in the following:
![image](https://user-images.githubusercontent.com/9696959/41525231-2dbd24f2-72d8-11e8-80a7-8075e5dc597f.png)

However, the SVG is a lot sharper. Doing a bit of [research](https://github.com/nfroidure/svgicons2svgfont/issues/85) and it seems that fontHeight should be the appropriate fix.

I modified the my locally installed version of [icons-to-woff.js](https://github.com/jantimon/iconfont-webpack-plugin/blob/master/lib/icons-to-woff.js#L19-L26) to pass a hard coded `fontHeight: 1000` as suggested by https://github.com/nfroidure/svgicons2svgfont/issues/85 and it fixed the issue.

![image](https://user-images.githubusercontent.com/9696959/41525394-bd58339a-72d8-11e8-91f8-5a7a416986d7.png)

It would be good to have the ability to pass along any other options that [svgicons2svgfont](https://www.npmjs.com/package/svgicons2svgfont) can accept so that it offers a lot more flexibility and does not require you to intervene.

Furthermore, if `fontHeight` is provided, then https://github.com/jantimon/iconfont-webpack-plugin/blob/master/lib/icons-to-woff.js#L54 can be removed as svgicons2svgfont automatically normalizes before needing to be passed through the subsequent libraries to get the woff file.