Add a slash to directories when listing

Having a slash after a directory name is yelling *DIRECTORY* for decades.

That's why I really like the `-F` flag.

It has other useful features as well.

From the `ls` manual:

> Display a slash (`/`) immediately after each pathname that is a directory, an asterisk (`*`) after each that is executable, an at sign (`@`) after each symbolic link, an equals sign (`=`) after each socket, a percent sign (`%`) after each whiteout, and a vertical bar (`|`) after each that is a FIFO.