Allowing format specific options

[mrbar42]

There is a use case where ts option needs to be passed down.
at the moment there is no way to pass options to the actual font generation module.

the PR adds a formatOptions object that allows passing arbitrary options to the specific format generator.

with the additions, the usage would be as follows:

var webfontsGenerator = require('webfonts-generator')

webfontsGenerator({
  formatOptions: {
    // options to pass specifically to the ttf generator
    ttf: {
        ts: 1451512800000
    }
  }
}, function(error, result) {})

[sunflowerdeath]

It looks useful. Thank you for improvement.

[mrbar42]

can you by any chance publish this to npm as a patch or a minor?

[sunflowerdeath]

im working on it right now )