Add woff2 mime type for inline_font_files

This fixes an error message when using inline_font_files with woff2 files: "A mime type could not be determined ... please specify one explicitly."

As far as I can tell, it is not actually possible to explicitly specify a mime type, as inline_font_files only passes the path argument to compute_mime_type. 

This adds woff2 to the list of mime types in compute_mime_type, which should eliminate an error when calling inline_font_files directly with woff2 files.