fix bug unescaped_string_literal

Running the command:

erlydtl:compile(<<"{{ text|add:\"\\\"\" }}">>, test).

will result in a crash. string:strip will remove all \" from the
beginning and end of the string \"\\\"\", leaving only \\. It should
only remove one \" from the beginning of the string and one from the end.

src/erlydtl_compiler.erl

@@ -1383,7 +1383,7 @@ spaceless_ast(Contents, Context, TreeWalker) ->
 	[Ast]), Info}, TreeWalker1}.
 
 unescape_string_literal(String) ->
-    unescape_string_literal(string:strip(String, both, 34), [], noslash).
+    unescape_string_literal(remove_quotes(String), [], noslash).
 
 unescape_string_literal([], Acc, noslash) ->
     lists:reverse(Acc);
@@ -1400,6 +1400,16 @@ unescape_string_literal("t" ++ Rest, Acc, slash) ->
 unescape_string_literal([C | Rest], Acc, slash) ->
     unescape_string_literal(Rest, [C | Acc], noslash).
 
+remove_quotes(String) ->
+    remove_last_quote(remove_first_quote(String)).
+
+remove_first_quote([34 | Rest]) ->
+    Rest;
+remove_first_quote(String) ->
+    String.
+
+remove_last_quote(String) ->
+    lists:reverse(remove_first_quote(lists:reverse(String))).
 
 full_path(File, DocRoot) ->
     case filename:absname(File) of