Support for Groups in Connections File

[orlovm]

Support for Groups in Connections File

This MR adds support for groups in the connections file and closes #135.
All changes are fully backward compatible with existing connection files.

New Structure

[
    {
        "name": "this_is_a_group",
        "connections": [
            {
                "url": "postgres://postgres:postgres@localhost:5432/postgres",
                "name": "this is a nested connection"
            }
        ]
    },
    {
        "url": "postgres://postgres:postgres@localhost:5432/users",
        "name": "users"
    }
]

Implementation Notes

No additional group manipulation commands were implemented.
Grouping is handled naturally by prefixing connection names with the desired group path when using :DBUIAddConnection.
Groups could be renamed and deleted as usual connection.

[kristijanhusak]

Thanks for the PR, and sorry for holding this for so long.
Can you rebase from master and fix the conflicts? There were some changes in the rendering logic from the previous PR.

[orlovm]

Done!
I waited two years for someone to implement this before doing it myself, so waiting for a review is no big deal.
BTW, this is one of my favorite and most used plugins—thanks for it!