Add export command

[RhetTbull]

Add notes export DIRECTORY to export all or some notes to DIRECTORY:

--json: export to JSON
--sqlite: export to SQLite DB
--txt: export to text
--html: export to HTML
--attachments: export attachments (link in HTML)
--folders: export by folder
--markdown: export as markdown

--filename TEMPLATE: use metadata template language template for specifying the file path, e.g:

--filename "{account}/{folder}/{title} - {date}"

[RhetTbull]

See also #20, could be done as --sqlite

[RhetTbull]

Also when exporting individual notes, the creation date and modification date of the file should be set

touch -t will change creation date
touch -mt will change modification date

Also add --no-touch option which doesn't change the dates.