Support encryption

[mattrighetti]

#39

Aim of this PR is to introduce two new commands to lock and unlock envelope.

For this first iteration, I'm only looking to introduce a way to de/encrypt envelope. This means that users will be able to lock the database but won't be able to perform operations on it while it is in its encrypted state. In the future I plan to make all the operations executable even if the database is in an encrypted state.

Example of what this PR will introduce:

$ envelope lock
Enter passphrase: ************
envelope is locked

$ envelope list
error: envelope is locked

$ envelope unlock
Enter passphrase: ************

envelope is unlocked

$ envelope list
prod
dev