Release v1.5.3

Since version 1.5.2, these changes have been made:

• Email Headers You can now edit ~/config/formulate/email.config to specify headers to add to your emails sent by the "Email" handler. For example, an "X-Mailer" or a "Message-ID" header. Thanks to @drooks for implementing this.
• Email Format Emails sent by the "Email" handler will now be sent with both HTML and plain text formats.