Add commands to streamline review and execution of a contract

[shonfeder]

A nice workflow might be something along the lines of the following:

neat-contract review <giturl>
neat-contract execute <contract-name>

Where review would take care of

1. downloading the contract at the url (assuming Add procelain to manage cloning, and committing to repos #6)
2. registering it in the known contracts (also assuming Add procelain to manage cloning, and committing to repos #6)
3. opening the contract for review (prompt for which contract, and prompt for last registered)

and execute would take care of

4. checking whether an id has been added
5. if not, prompt user to add appropriate ID
6. signing the contract with ID
7. compile the contract
8. commit and push the changes (assuming Add procelain to manage cloning, and committing to repos #6)

[shonfeder]

Add high-priority label, due to direct relevance to this workflow criteria from our alignment doc:

Input parameters and sign the contract in one execution.