feat: add ability to customize note file format

[dcchambers]

• Add NOTE_FORMAT configuration option. The default is markdown (.md)
• Newly created notes in the standard date format will end in this file extension.
• Newly created notes named with the -n option will default to this format only if they do not have a specified extension.
  • For example, note -n my-note.md will create the note my-note.md, and note -n my-other-note will create the note my-other-note.md