Preserve exact note content when exporting #111

hackmd-cli export currently adds a newline after the note content because it prints the content using the normal logging method.

As a result, redirecting an exported note to a file may change its content:

hackmd-cli export --noteId=<note-id> > note.md

The exported file should exactly match the content stored in HackMD so it can be safely edited and uploaded again.

Expected behavior:

• Write note content to stdout without adding or removing characters.
• Preserve content both with and without a trailing newline.
• Add tests for trailing newline, no trailing newline, and empty content.