Add initial version of the USEful script.
Add .gitignore.
Change used/unused into enabled/disabled.
Pull caching into global scope for the time being.
Check validity of EXPAND_USE flag passed as argument.
Factorize ugly regexp list to ease future cleanup.
Started implementing execute().
Formatting. Comments.
Corrections in comments. Remove unneeded default expansions in list_enabled() and list_disabled().