Fix for issue #80 (#81)

* Fix for issue #80

When updating the `opening_chars`, line_statment was forgotten. So, a
change to any of the openers would cause line statements to start being
unrecognized.

I added a unit test to cover this case.

* fix indentation

* add update to single_include

include/inja/config.hpp

@@ -25,7 +25,10 @@ struct LexerConfig {
   std::string open_chars {"#{"};
 
   void update_open_chars() {
-    open_chars = "\n";
+    open_chars = "";
+    if (open_chars.find(line_statement[0]) == std::string::npos) {
+      open_chars += line_statement[0];
+    }
     if (open_chars.find(statement_open[0]) == std::string::npos) {
       open_chars += statement_open[0];
     }