Store Dat config data, secret keys, etc. in default platform folders

[RangerMauve]

As mentioned on IRC (thanks @da2x ), it'd be good for Dat to avoid putting folders into the home directory and to instead follow the standards used by other applications like XDG.

We could make use of the platform-folders module in order to move the dat secret keys and other relevant storage somewhere that makes sense for the user's OS.

Some considerations:

• How will we migrate the data?
• What names should we use across platforms?
• Is this worth the breaking changes?

Also related to #1063

This will probably involve a change to dat-storage. Maybe we can have it search for ~/.dat as a fallback and create new keys in the appropriate dir.

[joehand]

Also may look at https://github.com/sindresorhus/env-paths and https://github.com/sindresorhus/conf