Feature Request: Allow exporting of all documents

[mynameismon]

It would be really helpful if one could export all documents with a flag.

For example:

hackmd-cli export --all --md

This way, one can easily keep their documents on their device as a backup (in case any loss of data occurs)

[morganpartee]

Agreed - Publish doesn't meet my needs so I need a way to fetch all docs

(or only changed docs, preferably... but I can dream haha.)