Before this change, rclone used the `__custom_func` hook to control
the completions of remote files. However this clashes with other
cobra users, the most notable example being kubectl.
Upgrading cobra to master allows us to use a namespaced function
`__rclone_custom_func` which fixes the problem.
Fixes#1529
Instead of showing all flags/backends all the time, you can type
rclone help flags
rclone help flags <regexp>
rclone help backends
rclone help backend <name>