Add 'emacs' alias for Emacs Lisp

Lets you use '#!/usr/bin/env emacs --script' shebang line.

lib/linguist/languages.yml

@@ -276,6 +276,7 @@ Emacs Lisp:
   lexer: Scheme
   aliases:
   - elisp
+  - emacs
   primary_extension: .el
   extensions:
   - .el