Inspect multiple indexes all stored in the same directory

[olliefreeman]

We use lucene to index multiple classes using hibernate search and as such we have the index root path of /opt/lucene inside which we have multiple index folders. It would be handy for us to be able to run Marple looking at /opt/lucene and have it display each of the indexes inside, rather than having to stop and restart Marple each time to look at a different folder.

We're happy to look into the code updates for a PR but would like some help as to where to start, unless this is something which could be implemented quickly by you.