Incremental addition of new files to grip index

[ToxicFrog]

At the moment, gripgen supports two modes:

• gripgen absent other flags creates a new index ex nihilo, discarding the previous index contents
• gripgen -u ignores the contents of stdin and reindexes every file referenced in the database, whether or not it has changed

The former works fine when you want to reindex everything, and the latter when you want to reindex everything and know the set of files to index hasn't changed. But neither is ideal when you know that that previously indexed files haven't changed, but you want to add new files to the index, e.g. datestamped logfiles.

An incremental mode, where files passed to gripgen are indexed without clearing the existing database, would solve this; the list of files would be easily generated via, e.g., a find command looking for files that have been modified more recently than the gripindex itself.