Add auto-chmod for better UX than "permission denied" for non-executable files

If running a file/script/binary in the CLI without the executable bit set, it will fail with "permission denied", which isn't clear about what happened and is annoying. Instead, have a small stub that asks the user if they want to make the file executable and run it, saving time and keypresses while being clearer about what is happening.

CC: @ngraham