fs: Add directory to optional Purge interface - fixes #1891

- add a directory to the optional Purge interface
- fix up all the backends
- add an additional integration test to test for the feature
- use the new feature in operations.Purge

Many of the backends had been prepared in advance for this so the
change was trivial for them.

backend/azureblob/azureblob.go

@@ -967,8 +967,7 @@ func (f *Fs) Hashes() hash.Set {
 }
 
 // Purge deletes all the files and directories including the old versions.
-func (f *Fs) Purge(ctx context.Context) error {
-	dir := "" // forward compat!
+func (f *Fs) Purge(ctx context.Context, dir string) error {
 	container, directory := f.split(dir)
 	if container == "" || directory != "" {
 		// Delegate to caller if not root of a container