Automatically infer @font-face src format

- The `format` string in CSS `@font-face` is now automatically inferred from the file type, so `---font-format` is now redundant. I've followed the [CSS spec for `src`](http://www.w3.org/TR/css3-fonts/#descdef-src).

- The existing code confused mime type with CSS src format - these are not the same, so there are now two separate strings `mime_type` and `format`.

- I've deprecated the `---font-format`  rather than removing it. It issues a nice custom message explaining why.