Configure backup location

[maaaat]

Is your feature request related to a problem? Please describe.
The backup option drops the files at the root of the internal storage (/storage/emulated/0/skytube-*.skytube) and includes the date in the filename. That's not very convenient to save to external storage (e.g. WebDAV) or to add to backup tools (e.g. Seedvault).

Describe the solution you'd like
It would be great to be able to specify a folder where to store the backup files.

Describe alternatives you've considered
Now I have to remember to copy the backup file by hand every time.