Persistent history

Command history is handy, but it doesn't live beyond a session. It would be awesome if `vantage` had persistent history. As of now, this can only be achieved by loading entries into `vagrant.session._hist`.

History could be kept server-side on a per-user basis, or client-side on a per-host basis.

For a server-side implementation, instead of bundling this functionality with `vantage` itself, a public history API could be exposed for extensions to manage and provide history, with session history as the default implementation.

A client-side implementation could provide a `vantage` command-line flag to pick a history file, defaulting to something like `~/.vantage/history/` (for system-wide, per-host history) or `./vantage/history` (for directory-based history).