Added rclone special remote, which can be used without needing to install
the git-annex-remote-rclone program. This needs a new version of rclone,
which supports "rclone gitannex".
This is implemented as a variant of an external special remote, that
runs "rclone gitannex" instead of the usual git-annex-remote- command.
Parameterized Remote.External to support that.
Sponsored-by: Luke T. Shumaker on Patreon
