Add syntax highlighting for jinja/nunjucks

Add syntax highlighting for jinja/nunjucks, map to set filetype to
htmldjango (leader+d). syntax highlighting set with autocmd, as it
doesn't seem to work automatically when setting the filetype.
Also added augroups and !autocmd to prevent resetting of autocommands
repeatedly when resourcing .vimrc.
Todo: try autocommand change colors of linenumbers fground and bground.
This also only works when sourcing vimrc. Very annoying.