Force user to choose download directory

[DailyDreaming]

If a user deletes their downloaded files they may be under the false impression that they've regained the disk space.

One possible solution is to add a new option that tells it whether to clean up or not.

[hannes-ucsc]

My goal was always to allow successive invocations of the download functionality to reuse the file store. Also note that there are two possible implementation choices if we decided to delete the file store by default at program exit: always and on success only. The former would prohibit resumptions and is therefore unacceptable in my opinion. The latter would reintroduce the possibility of the file store to take up space without the user noticing.

One option not previously discussed is requiring the user to specify the download directory. The invariant would be that the file store is always a subdirectory of the download directory. If the specified download directory does not exist it would be created. This would signal more clearly to the user that they would need to delete that entire directory (which includes the file store) in order to "undo" the effect of a download.

[kozbo]

I prefer the option of requiring the user to specify the download directory. I think it would help to add a little documentation to this to tell users how to clean up.

[hannes-ucsc]

Let's go with those things two measures then, @kozbo.

I gave some additional reasoning here as well #394 (comment).

[theathorn]

Also see #397.