Just as a side note:
As I only need the syntax and indentation for gohtmltmpl from vim-go (just two files) I use this simple script to detect the filetype:
function DetectGoHtmlTmpl()
if expand('%:e') == "html" && search("{{") != 0
set filetype=gohtmltmpl
endif
endfunction
augroup filetypedetect
au! BufRead,BufNewFile * call DetectGoHtmlTmpl()
augroup END
Checks if file extension is html and if file contains search term {{
Maybe someone can use this.