Add support for custom icons

[jdewaen]

Font Awesome allows users with a "Pro" subscription to upload custom icons which will include those along with the stock icons. When the icon pack is downloaded, they are included as a style named "custom-icons":

When this custom-icons.svg sprite is placed in the sprites directory as instructed in the README, they are ignored entirely. This can be fixed by adding an entry to mappings at IconsetEnumGenerator:110. I would propose fa-kit as the key, to match the css class used by Font Awesome to target these custom icons. Alternatively, you could use fak, to keep things more in line with the existing keys.

Adding the extra style has one side effect: the Enum name for the Java class is based on the filename of the sprite. However, Java doesn't support hyphens in class names. To allow for easy copy/pasting of these sprites, the code should transform kebab-case into PascalCase automatically when generating the name of the enum.