convert: allow specification of tmp directory location

[astralarya]

I keep my music library on an external RAID array, but /tmp is located on a separate file system. It would be nice to be able to specify the location where beet places the temporary files, which would allow it to be on the same file system. This would allow for a move operation to place the file into the beet library, as opposed to the copy/delete required when transferring between file systems.