Support external storage locations for computed media columns

[mkornacker]

At the moment, all computed media values are stored in ~.pixeltable/media.

Instead, the user should be able to specify a storage destination when creating the table/column, like so:
create_table('new_tbl', {'video': VideoType(), 'audio': {'type': AudioType(), destination='/path/to/local/dir'}})

In addition, this should also support an external URL as the storage directory parameter (ex.: 's3://my-image-bucket...')