Highlight Elixir string markdown header correctly

Elixir allows markdown inside its triple-quote heredoc syntax. However,
the "#" used in markdown headers is currently misinterpreted as a
comment. So, if you write this:

```elixir
@doc """
Get the first name of a user.

- `user` - A User struct.

    user = %User{name: "Alice Winston"}
    User.first_name(user)
    "Alice"

"""
```

Prism interprets the markdown headers as comments, not as part of the
string, and this messes up the coloring.

This can be fixed by adding a negative lookbehind, such that the
comments regex matches "#" but not "##".

components/prism-elixir.js

@@ -1,6 +1,11 @@
 Prism.languages.elixir = {
 	// Negative look-ahead is needed for string interpolation
-	'comment': /#(?!\{).*/,
+  // Negated capture group is needed to avoid highlighting markdown headers in
+  // multi-line doc strings
+  'comment': {
+      pattern: /([^#])#(?![{#]).*/,
+      lookbehind: true
+  },
 	// ~r"""foo""", ~r'''foo''', ~r/foo/, ~r|foo|, ~r"foo", ~r'foo', ~r(foo), ~r[foo], ~r{foo}, ~r<foo>
 	'regex': /~[rR](?:("""|'''|[\/|"'])(?:\\.|(?!\1)[^\\])+\1|\((?:\\\)|[^)])+\)|\[(?:\\\]|[^\]])+\]|\{(?:\\\}|[^}])+\}|<(?:\\>|[^>])+>)[uismxfr]*/,
 	'string': [
@@ -81,4 +86,5 @@ Prism.languages.elixir.string.forEach(function(o) {
 			}
 		}
 	};
-});
+});
+