Add subtitle file voicing feature, improvements in code and documentation

abogen/queued_item.py

@@ -14,6 +14,8 @@ class QueuedItem:
     output_format: str
     total_char_count: int
     replace_single_newlines: bool = False
+    use_silent_gaps: bool = False
+    subtitle_speed_method: str = "tts"
     save_base_path: str = None
     save_chapters_separately: bool = None
     merge_chapters_at_end: bool = None