Drush commands for test email and delete all logged emails

Problem/Motivation

It would be nice to have a way to delete all test emails from the command line via drush. It might also be nice to send a test email from the command line, and others may think of additional commands and want to expand upon these as well.

Since I got it in the other ticket... might as well add a command that confirms the configuration is right for this module. Why not!

Steps to reproduce

Proposed resolution

Two drush commands:

drush dml:send-test-email
drush dml:clear-email-log
drush dml:confirm-config

Remaining tasks

User interface changes

API changes

Data model changes