Expose codepoints via SCSS variables

The allows easily creating custom icon elements from SCSS that includes the generated file.

For example, given a font name of "my-icon-font" with an icon named "user", the icon can be added to any selector:

```scss
#my-custom-el {
  font-family: "my-icon-font",
  content: $my-icon-font-user;
}
```