Allow to run against local Backup of USDB as alternative source for USDB

[mjhalwa]

I think of a simple backup of usdb within a directory that contains all txt files, each coupled with the song's detail page. In case of usdb downtimes it would be nice to operate the syncer against this directory instead of usdb.

E.g. say we have a directory "USDB" with content:

• list.html
• 00012.txt
• 00012.html
• 00015.txt
• 00015.html
• ...

where list.html is the result page from usdb search without any search criteria and Limit=100k (big enough) and the other .html files are each song's detail page.