Support Erb Files Too

In Rails we can have files like "foo.hbs.erb", which, at this point, aren't supported by this plugin. This change was available in nono/vim-handlebars (see https://github.com/nono/vim-handlebars/blob/f419dd0fe0f8df1712285a06260d809dd343f776/ftdetect/handlebars.vim#L1-L3). This commit brings this change to this plugin.