Generate download filename

[bennettmcelwee]

Generate the download filename from existing text (e.g. "Recent searches") which will simplify things for translators.

1. Get the localized text
2. Apply a filter (so it can be changed by plugins/functions)
3. Sanitize it (https://codex.wordpress.org/Function_Reference/sanitize_file_name)
4. (Don't do this) Lowercase_ it ($str = mb_strtolower($str, mb_detect_encoding($str));)