WIP on ultralist 2.0

[gammons]

This is a WIP on v2.0 of ultralist.

The main goal of this is to bring a higher level of professionalism and structure to the project as a whole. This will be achieved by:

1. Splitting concerns into packages. There will be a core ultralist pacakge, which will implement the main business logic around Todo lists, todos, recurrence, etc.

2. Eliminate dependence on CLI. Ultralist will eventually have a more tui-like user interface, and there are a lot of places in the app that have a reliance on ultralist being a CLI only. Separate the concerns of CLI interaction into a cli package to prepare for that.

3. Proper use of errors. Improve error handling across all packages.

4. Prepare for a re-think of the backend + syncing. Ultralist should use a store like CouchDB for syncing. Prepare the app for that eventuality.

[tcassaert]

With point 2, would that mean there is no way to use it as a simple CLI anymore? Or will there be the CLI interface and the TUI interface?

[gammons]

@tcassaert there will be both! None of the existing functionality will go away.