v7.8.0

Add action props of box component and maxLines props of text component #174 (@hinoguma )
Add default action to button template and carousel template #211 (@hinoguma )
Messaging API - December 2020 update : Add keywords property to Sticker event #245 (@clsung )
Messaging API - January 2021 update : Add mention props for text message #247 (@hinoguma )
Messaging API - January 2021 update : add acceptedTime and completedTime to MessagesProgressResponse #247 (@hinoguma )