Properly handle invalid UTF-8 in filenames

[sharkdp]

In particular, OsStr to String conversions may fail.

Quote by @BurntSushi on reddit:

You are converting all paths to strings before searching, which will probably blow up on you in some cases when file paths don't contain valid UTF-8. (And it does happen, because I had a similar bug in my own code.) On *nix at least, you can extract the raw &[u8] from a file path safely and then feed that into a regex::bytes::Regex (instead of regex::Regex).