add export command for CSV export of vault

Add a new export command that allows batch exporting of the vault
in CSV format, which can be re-imported via a LastPass plugin.  The
output is unencrypted but can be re-encrypted by piping to gpg or
similar.

Signed-off-by: Evan Johnson <evan@lastpass.com>
Signed-off-by: Bob Copeland <copeland@lastpass.com>