Save the token to ~/.vault-token for compatibility with other tools

[onitake]

The official vault client and other tools (such as the Ansible module) expect the auth token in ~/.vault-token.
It would be useful if vault-client could store the token into this file as well instead of the config file.
This would remove the manual step of copying the token over.

[winged]

👍 This would also help immensely when you're managing your dotfiles via VCS. Otherwise, your config shows up as changed every time you get a new token.