Change --mode to --what
Update constant management.

* Updated private helper method names.
* Management of constants is now made from a static class property
    * Required, Optional, and Copy from policies are supported
* Updated %ENV%_PATH to %ENV%_SSH_PATH for consistency.
* %ENV%_SSH_DB_HOST, %ENV%_SSH_DB_USER and %ENV%_SSH_DB_PATH are optional as
they are duplicated from %ENV%_SSH_HOST, %ENV%_SSH_USER, and %ENV%_SSH_PATH
respectively.
Remove old empty help.
Add support for 'wp help deploy push'
Update readme.