when a tiddler title is changed, node.js server version needs to cleanup...

... a file with the previous tiddler title

I'm using TW5's node.js server configuration. Whenever I edit and existing tiddler and change the title, I end up with two tiddler files. I'm not sure if this issue would affect any other TW5 configuration, but it didn't seem to apply to a single HTML file in my quick tests.

### Steps to Reproduce
* Using the node.js server configuration
* Create and save a tiddler with title "Test One"
* Edit and save the tiddler changing the title to "Test Two"
* There'll be two tiddler files remaining. If you refresh the wiki, you can find both tiddlers are loaded.

The included changes in the filesystem adapter do seem to correct this issue, they delete the original tiddler file when the Draft of ... is deleted based on the `draft.of` and `draft.title` fields. However, as I'm new to the TW5 code base, I'm not sure if there would be a better way to resolve this issue.