Feature: Sync local address books

[jhit]

Hello.

I miss the possibility to sync local address books. I want to sync my contacts via CardDav from one account an mirror them to Outlook.com or GMail.
TbSync can sync both accounts, but it cannot sync the two address books locally.
If I could set up a local sync for Address Books, I additionally could solve the problem described in #262 if I can locally sync the Outlook.com address book to Thunderbirds primary PAB.

Making a local address book sync possible TbSync could grow into an address book and calendar sync hub. Such services are available online and are quite expensive.

[jobisoft]

https://github.com/jobisoft/TbSync/wiki/How-to-sync-an-existing-Thunderbird-calendar-to-a-remote-account