Introduce wp db users #138

One of the most common tasks database administrators need to do is create user accounts for applications with full privileges to the application's database. I propose that we make that super simple in wp-cli using syntax similar to:

wp db users create --user --host --pass --privilegeduser --privilegedpass

Name of the database user to create.
Hosts that are permitted to connect as .
New user's password.
Database user with sufficient privileges to execute the command.
Privileged user's password.

This create command could be augmented by including list, delete, etc.