Implement top-level (list) sync in cvcap cli

[crispinb]

Not sure if cli will ever use sync proper (which is mainly intended for future apps using cvapi lib)
But a useful slice as a POC would be:

on cvcap add task -l, read the lists from sqlite so they're instantly available.
Add a 'synclists' command to refresh the local list.
Not really a sync but just a list download-and-persist.

[crispinb]

• add api to download and persist lists to local sqlite
• add api to get lists from sqlite rather than REST
• add synclists command to pop. file database
• use persistence api in all commands using -l option