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/amazonclouddrive/amazonclouddrive.go

@@ -937,8 +937,8 @@ func (f *Fs) Hashes() hash.Set {
 // Optional interface: Only implement this if you have a way of
 // deleting all the files quicker than just running Remove() on the
 // result of List()
-func (f *Fs) Purge(ctx context.Context) error {
-	return f.purgeCheck(ctx, "", false)
+func (f *Fs) Purge(ctx context.Context, dir string) error {
+	return f.purgeCheck(ctx, dir, false)
 }
 
 // ------------------------------------------------------------