file path filter tweaks, fixes #254

* Append trailing slash to `path` so that `_posts/a` directory path will no longer match `_posts/articles` (displaying `rticles`).
  - Should trailing slash only be appended if not already present?
* Only mark filenames if `searchstr` has a length.
  - Should mark be `<strong></strong>` instead of `<b></b>`?