export-loom should be able to ignore bottom thread (at least)

[Barry Warsaw]

export-loom is very cool, however I name my bottom loom 'rocketfuel'. I'd like to be able to export all my threads to my main Launchpad development directory, but I wouldn't want export-loom to touch my normal rocketfuel mirror branch, which is also called 'rocketfuel'.

Right now I export into a subdirectory, which is workable, but it would be nice if export-loom could be taught to ignore certain threads, or even just the bottom thread. I suppose bug 194274 has some relationship to this bug, though it seems to me it's aiming at a different use case.