Transcode to intermediate directory rather than output directory.

[Shelton Koskie]

This is a small request that will enable users to set an automator folder as destination directory.

Right now, Subler transcodes from the source directory into the destination directory, and then updates/optimizes the transcoded file by creating an mp4.tmp file and then replacing the original mp4 with that one. This prevents users from creating an automator folder to act on the converted file.

Instead, it would be helpful if you transcoded to a hidden temp (src_dir/.tmp/) directory, performing the optimization there, and moving to the destination directory only when the entire operation is complete. The destination directory can then be set as an Automator folder that will perform additional actions.

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This issue is slightly related to #128, but that one relates to using Automator to initiate file processing with Subler, whereas this issue relates to how files are handled after processing is already initiated.

Thanks for all the work you put into this awesome app. It's greatly appreciated.