Config store

[joehand]

• We don't need this module: https://github.com/datproject/dat-store/blob/master/client.js#L5, its part of node core now.
• Use XDG base directory when possible

(Also we should evaluate options for https://github.com/datproject/dat-secret-storage/blob/master/index.js#L2, since that is also deprecated)

[RangerMauve]

Covered in #8