Draft of exporting feature, refs #289

note export --txt:  Export everything as txt files
note export --json: Export everything as json files

Missing: export KEY --tyt: Export a note as txt

Signed-off-by: Stefan Tatschner <stefan@sevenbyte.org>
Implemented an export command, refs #289.

Signed-off-by: Stefan Tatschner <stefan@sevenbyte.org>