Omit FontName from Icon Class?
Hey there!

I started using `svgtofont` today, and I'm really enjoying it so far.

One quick question: I have an existing icon set that I'm migrating over to use `svgtofont` as a backend for set generation. One thing I'm not totally sure about is that the current output includes the font name in the class. So, with this setting:

```
  fontName: "TestFont", // font name
  classNamePrefix: "tf", // class prefix
```

Each icon in the font output has a class like `TestFont-tf-icon`. However, I want to maintain compatibility with my old icon set to avoid breaking changes.

How might I keep the fontName as TestFont, but omit it from the icon classes? For compatibility with the old set, and to serve as a drop-in replacement, I'd like each icon to only have the class `tf-icon` instead.