Add th-saving-tiddler hook for autotagging etc.

@danielo515 this enhancement is intended to make it easier to implement
things like the auto tag plugin. Does it meet your needs?

editions/dev/tiddlers/new/th-opening-default-tiddlers-list.tid

@@ -1,6 +1,6 @@
 created: 20141122200310516
 modified: 20141122200310516
-title: Hook: tc-opening-default-tiddlers-list
+title: Hook: th-opening-default-tiddlers-list
 type: text/vnd.tiddlywiki
 
 This hook allows plugins to add to or remove from the list of tiddlers that are opened when the wiki is first loaded or the home button is clicked.

editions/dev/tiddlers/new/th-saving-tiddler.tid

@@ -0,0 +1,22 @@
+created: 20150908150314994
+modified: 20150908150314994
+title: Hook: th-saving-tiddler
+type: text/vnd.tiddlywiki
+
+This hook allows plugins to modify tiddlers before they are saved via the ''confirm'' toolbar button; the hook is not invoked for tiddlers that are saved through other means, such as state tiddlers created by the ActionSetFieldWidget.
+
+Hook function parameters:
+
+* ''tiddler'': tiddler object about to be saved
+
+Return value:
+
+* tiddler object to be saved
+
+The original tiddler object can be returned unmodified by the hook. If the hook needs to modify the tiddler then it should return a new tiddler object, for example:
+
+```
+	return new $tw.Tiddler(tiddler,{"my-field": value});
+```
+
+Hooks must not change the ''title'' field but can freely modify any other field of the tiddler.