Make all config file variables be settable in the environment

These are set in the form RCLONE_CONFIG_remote_option where remote is
the uppercased remote name and option is the uppercased config file
option name.  Note that RCLONE_CONFIG_remote_TYPE must be set if
defining a new remote.

Fixes #616

fstest/fstests/fstests.go

@@ -71,7 +71,7 @@ func TestInit(t *testing.T) {
 	fs.LoadConfig()
 	// Set extra config if supplied
 	for _, item := range ExtraConfig {
-		fs.ConfigFile.SetValue(item.Name, item.Key, item.Value)
+		fs.ConfigFileSet(item.Name, item.Key, item.Value)
 	}
 	fs.Config.Verbose = *verbose
 	fs.Config.Quiet = !*verbose