XL: Easy datastore (permanent or on tmpfs)

[christianTF]

For reoccurring requests, an easy datastore should be possible.

It could follow the implementation from https://raw.githubusercontent.com/mschlenstedt/Loxberry/master/webfrontend/html/XL/examples/gruenland.php

Features:

• Selectable if the datastore is persistent (sd card) or temporary (tmpfs)
• One call to open or create the datastore and present the data in an object
• Automatic save on script exit
• Explicit function to rollback changes (do not save)
• Filenames are automatically created and unique and only an optional parameter

This could follow the system of LoxBerry::JSON, but simplified.