Update description for format.formatter

[ryanfitzer]

Description of changes:

Make it clear that the function is bound to the file object.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[ryanfitzer]

I think this will help to document the current behavior.

For a future release, I think the current file object should be passed to the function, instead. Example (avoids a breaking change):

StyleDictionary.registerFormat({
  name: 'json',
  formatter: function(dictionary, config, file) {
    return JSON.stringify(dictionary.properties, null, 2);
  }
})

[dbanksdesign]

LGTM!

I like your suggestion about having the file object as a parameter rather than binding the context to it... I forget why we did that in the first place...

[ryanfitzer]

Any changes needed to get this merged?

[chazzmoney]

LGTM!