fs: Allow on the fly remotes with :backend: syntax - fixes #2449

This change allows remotes to be created on the fly without a config
file by using the remote type prefixed with a : as the remote name, Eg
:s3: to make an s3 remote.

This assumes the user is supplying the backend config via command line
flags or environment variables.

docs/content/docs.md

@@ -133,6 +133,48 @@ It is recommended to use `copy` when copying individual files, not `sync`.
 They have pretty much the same effect but `copy` will use a lot less
 memory.
 
+Syntax of remote paths
+----------------------
+
+The syntax of the paths passed to the rclone command are as follows.
+
+### /path/to/dir
+
+This refers to the local file system.
+
+On Windows only `\` may be used instead of `/` in local paths
+**only**, non local paths must use `/`.
+
+These paths needn't start with a leading `/` - if they don't then they
+will be relative to the current directory.
+
+### remote:path/to/dir
+
+This refers to a directory `path/to/dir` on `remote:` as defined in
+the config file (configured with `rclone config`).
+
+### remote:/path/to/dir
+
+On most backends this is refers to the same directory as
+`remote:path/to/dir` and that format should be preferred.  On a very
+small number of remotes (FTP, SFTP, Dropbox for business) this will
+refer to a different directory.  On these, paths without a leading `/`
+will refer to your "home" directory and paths with a leading `/` will
+refer to the root.
+
+### :backend:path/to/dir
+
+This is an advanced form for creating remotes on the fly.  `backend`
+should be the name or prefix of a backend (the `type` in the config
+file) and all the configuration for the backend should be provided on
+the command line (or in environment variables).
+
+Eg
+
+    rclone lsd --http-url https://pub.rclone.org :http:
+
+Which lists all the directories in `pub.rclone.org`.
+
 Quoting and the shell
 ---------------------