Multiline support

This fixes multiline value support and tries harder to keep spacing in arrays.

In order to achieve that, this code makes use of `text_store`/`text_retrieve` with `Shellvars_list` when available.

The code for `in_sync?` in the type is also changed to properly manage multiline entries.

This requires the new `whichquote` method from core (https://github.com/hercules-team/augeasproviders_core/pull/2)