Add support for adding headers

[ansel1]

Headers are optional attributes removed from the end of the line, and injected (values only) into the header of the line, after the level/source, and before the message.

Addresses #6

• [x] tests
• [ ] polish the godoc
• [x] finalize the names
• [x] add a new property to Themes for the header, or just re-use Source(), since that is effectively part of the header?