Generating the code for methods Beautifier.Template() and Beautifier.Data() will ease theme management. This way Beautifer.Assets() can be used to simply define the required assets to build a theme.

• Remove implementions of Beautifier.Template() and Beautifier.Data() for the golang (default) theme
• Write a Go generator in pkg/themes/generator.go that will generate a default_gen.go file implementing those two methods with proper content taken from Assets()
• Update the build system to go generate before building