use modified time for update date_range
complete (untested) incremental update
fix bugs discovered during testing

`--use-cache` is now invalid, since no setting both discovers new files and does incremental updates.
The setting and its associated logic will probably have to be completely redone.
pass tests
update CLI help and README
be more explicit about looping of dict keys
simplify maintenance operations
Merge branch 'master' into incremental-updates