psite-dl-backup destination param should handle absolute file paths

[donutdan4114]

Currently, using psite-dl-backup, the destination parameter only works when specifying a directory. You cannot specify an output filename. This means that every time you download the backup, it will have a new name (unpredictable).
> drush psite-dl-backup $SITE_NAME dev latest database ./destination

If the command returned/output the filename of the file downloaded, that'd be a good start.

Suggested fix: if destination parameter is a file name (includes a ".") the file should be saved with that name. Or perhaps you can add a flag, for marking something as a file.
> drush psite-dl-backup $SITE_NAME dev latest database ./my-backup.sql

OR, if no destination param, output the file using stdout for more control
> drush psite-dl-backup $SITE_NAME dev latest database > ./my-backup.sql

[joshkoenig]

Seems legit. I believe all that would be required is allowing the user to pass a specific value here:

https://github.com/pantheon-systems/terminus/blob/master/terminus.drush.inc#L2099

Pull requests welcome, otherwise we'll get to it in an upcoming sprint.

[mikevanwinkle]

+1