When using `rclone cat` to print the contents of several files, the
user may want to inject some separator between the files, such as a
comma or a newline. This patch adds a `--separator` option to the `cat`
command to make that possible. The default value remains an empty
string, `""`, maintaining the prior behavior of `rclone cat`.
Closes#6968
This is possible now that we no longer support go1.12 and brings
rclone into line with standard practices in the Go world.
This also removes errors.New and errors.Errorf from lib/errors and
prefers the stdlib errors package over lib/errors.
