Tokenizer line numbers

Line numbers were added to the Liquid parser, this brings Liquid::C back in sync. Also removed the variable parsing.

@dylanahsmith 
```C
tokenizer->use_line_numbers = false;
if (argc == 2) {
  VALUE options = argv[1];
  Check_Type(options, T_HASH);
  tokenizer->use_line_numbers = RTEST(rb_hash_aref(options, ID2SYM(rb_intern("line_numbers"))));
}
```
seems cleaner to me - it's exactly the same logic without the temporary variable...