Preserve non utf8 filenames

[sharkdp]

This PR fixes the handling of filesystem entries with invalid UTF-8 names (#295). It is an adapted version of #309 by @alexmaco (thank you for your initial work!).

Invalid UTF-8 filenames are properly passed to child-processes when using --exec, --exec-batch or --list-details:

There are a few things to note:

• fd -x echo will print the actual invalid UTF-8 sequence, as expected
• if you call fd -x echo, you might still see "test_�invalid.txt" if your terminal emulator wants to save you from scrambled output.
• fd -l properly passes the filename to ls
• fd … -x rm properly passes the filename to rm

If not using -x or -X, we still make sure to print the "invalid UTF-8 sequence" character "�". However, if we pipe the output to another program (like xargs), we pass the filenames as is (on Unix):

> fd
test_�invalid.txt

> fd | xargs ls
'test_'$'\376''invalid.txt'

fixes #295