Implement new storage engine assumed to syncing with Dropbox

[yuuki0xff]

New storage engine assumed to syncing with Google Drive, Dropbox, or similar services. This feature is planned to replace the notes_as_txt feature.

Requirements:

• Save all notes into a specified directory.
• Data format should be human readable like plain text, markdown, reStructuredText, etc.
• Files may be modified by other processes.

Consideration:

• How to embed metadata like creation date, tags, and pinned.
• How to implement