config: make config file system pluggable

If you are using rclone a library you can decide to use the rclone
config file system or not by calling

    configfile.LoadConfig(ctx)

If you don't you will need to set `config.Data` to an implementation
of `config.Storage`.

Other changes
- change interface of config.FileGet to remove unused default
- remove MustValue from config.Storage interface
- change GetValue to return string or bool like elsewhere in rclone
- implement a default config file system which panics with helpful error
- implement getWithDefault to replace the removed MustValue
- don't embed goconfig.ConfigFile so we can change the methods

cmd/listremotes/listremotes.go

@@ -41,7 +41,7 @@ When uses with the -l flag it lists the types too.
 		}
 		for _, remote := range remotes {
 			if listLong {
-				remoteType := config.FileGet(remote, "type", "UNKNOWN")
+				remoteType := config.FileGet(remote, "type")
 				fmt.Printf("%-*s %s\n", maxlen+1, remote+":", remoteType)
 			} else {
 				fmt.Printf("%s:\n", remote)