Make the from email address filterable

By default backupwordpress send emails from the email address stored in the admin_email option.

The issues is some servers only allow sending emails from the current domain, in those cases the emails will fail to send.

Let's give those users a way to work around the issue by making the from email address filterable.
Merge pull request #434 from humanmade/filterable-from-email

Make the from email filterable