Add Header information to PDF

[ajquick]

This is a pull request to add the feature requested here: #27

The following ENV variables have been added:

EMAIL_HEADER EMAIL_HEADER_EXT

These are both False by default.

By turning on the Header, you will now have some basic information added to the PDF such as the subject, date/time, sender and recipient. Example:

Right now it displays:

• Subject
• Date
• From
• Reply-to
• To

Additional fields such as CC / BCC should probably be added.

By turning on the Header (extended) it will display the full headers as received. However, it currently does not display them very cleanly. Maybe it would be worthwhile to look into how to make them display better. It could be as simple as adding a <pre> tag.

Tested and works.

[ajquick]

@rob-luke Any idea on what needs to be reformatted?