Use a real database for storing, and retrieving plans

[tommartensen]

No decision whether relational or document DB has been made yet.
At the moment, there is an abstraction layer in https://github.com/tommartensen/tfui/blob/master/pkg/store/adapter.go that defines the methods that need to be implemented by a plan store, e.g. the file store: https://github.com/tommartensen/tfui/blob/master/pkg/store/file.go.