Allow us to specify the format

[elboletaire]

In gulp-iconfont we can specify the formats, but due to how gulp-iconfont-css generates the font-face code block we need to specify all the formats to avoid issues in some browsers.

Personally, I would like to just generate the woff and woff2 files, but right now I can't due to the hardcoded font-face block.

[iamso]

That or event better, read formats from gulp-iconfont.

[jefsnare]

@elboletaire I've added this also in PR #48 as comment