Add optional "from" option that can be set when sending test email

[mountaindude]

When using a smtp server that requires login to send test emails from Butler, the email will automatically be sent from the logged in user.

In other cases smtp servers don't require authentication/login, there is then a risk the email will not be sent as it lacks a proper sender.

Add an optional command line option that allows for specifying which sender email address should be used.