Add intro to create, make commands clear

[joehand]

Create gives a little intro (similar to desktop app) and prompts for title + description to add to dat.json (it defaults to anything already in dat.json).

❯ dat create
Welcome to dat program!
You can turn any folder on your computer into a Dat.
A Dat is a folder with some magic.

Your dat is ready!
We will walk you creating a 'dat.json' file.
(You can skip dat.json and get started now.)

Learn more about dat.json: https://github.com/datprotocol/dat.json

Ctrl+C to exit at any time

> Title (DEMO) 
> Description My dataset

Dat created! Now you can add files and share:
* Run dat share to create metadata and sync.
* Copy the unique dat link and securly share it.

dat://91d70ae66ecbf5893228ca8c9414c97418832cb1b5675879027422da6463b5ca

You can skip the intro/prompts with dat create -y.

[joehand]

@Karissa thanks for the suggestions, I reworked this PR to only be the new dat create intro. I also added a message to the header on dat share saying if it is a new dat:

❯ dat share
Created new dat in /Users/joe/dats/DEMO/.dat
dat://70b33958d0f40d4e4bfc0b60523c9ce8b8311b6966a4db0f22faa8808b158a4d
Sharing dat: 17 files (54 MB)

[okdistribute]

nice!