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.

fstest/fstest.go

@@ -482,7 +482,7 @@ func Purge(f fs.Fs) {
 	if doPurge := f.Features().Purge; doPurge != nil {
 		doFallbackPurge = false
 		fs.Debugf(f, "Purge remote")
-		err = doPurge(ctx)
+		err = doPurge(ctx, "")
 		if err == fs.ErrorCantPurge {
 			doFallbackPurge = true
 		}