Enable outputFilename with built in templates.

[funyapu]

https://github.com/itgalaxy/webpack-webfont/blob/b78f1c2d09ae6f767172ddd41b557c4a7be922f6/src/WebfontPlugin.js#L73

Please let developer rename output css with default css template.

I wanted to use the css in stylus, and so trying to rename file with "_" prefix, though it's been limited.

[alexander-akait]

@funyapu PR welcome

[emkayy]

I'm trying to generate 2 different icon fonts for a shared repository. However, with a custom template the template / font name is used as file names. Thus overwriting the previously generated font files. I was using grunt-webfont before, which offers a filename config option.