Move imprint.xhtml to the front of epub books.

Author: ihongda

.env_public

@@ -39,6 +39,7 @@ EDITION_FINDER_PATTERNS = '[
 ACCEPTED_REFERENCES = '[]'  # Sample: '["bj", "pts-vp-pli"]'
 ADDITIONAL_HEADINGS = '{
     "frontmatter": [
+        ("imprint", "Imprint"),
         ("foreword", "Foreword"),
         ("preface", "Preface"),
         ("general_introduction", "General Introduction"),