Support encryption of ~/.vault-token #79
Description
With #78 we will need to manually check to see if the token is stored on disk.
This opens up the ability to support encryption of ~/.vault-token to protect a stored token that has a long TTL.
Using vault login -no-store has the downside of forcing each new tty session to authenticate, which affects productivity when context switching between numerous connections. Allowing the token to be stored has the downside that it can be read, or accidentally stored in a backup/dotfile repo, etc.