When merging, start indices at 1 instead of 0.

When going through the resulting merged book in sigil, it's
confusing to remember that the index prefix on every file is not
actually the number of the book that you're working on, especially
if it's a numbered series of books.

epubfile.py

@@ -1325,6 +1325,8 @@ def merge(input_filepaths, output_filename, do_headerfile=False):
     rand_prefix = random_string(3, string.digits)
 
     for (index, input_filepath) in enumerate(input_filepaths):
+        # Number books from 1 for human sanity.
+        index += 1
         print(f'Merging {input_filepath.absolute_path}.')
         prefix = f'{rand_prefix}_{index:>0{index_length}}_{{}}'
         input_book = Epub.open(input_filepath)
@@ -1364,7 +1366,7 @@ def merge(input_filepaths, output_filename, do_headerfile=False):
             book.add_file(headerfile_id, headerfile_basename, content)
 
         for id in manifest_ids:
-            new_id = f'{rand_prefix}_{index:>0{index_length}}_{id}'
+            new_id = prefix.format(id)
             new_basename = basename_map[id]
             book.add_file(new_id, new_basename, input_book.read_file(id))