Reverse-sorted var keys fixes #21

Issue fixed: https://github.com/coderhaoxin/gulp-file-include/issues/21

- Sort passed variable keys in `include(file, text)`
- Traverse data keys in reverse-order
- Longer key names are encountered first, thus removing greedy replace on `title` as opposed to `title_link`, as mentioned in the issue #21
- Added tests to demonstrate functional improvement