add ability for tw to override savings setting

[buggyj]

This is for possible future use. Allow the settings to come from a tiddlywiki, base dir backup dir backup filename. This way a tiddlywiki plugin can implement backup protocol.

[buggyj]

the location of the base directory (set to tiddlywikilocations in the extension) and be overridden by the attrib data-tiddlyfox-base.

The backup sub-path and backup name can similarly be overridden