Feature Request: Allow exporting of all documents

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)

The requested entry point is `hackmd-cli export --all --md`; start by tracing the existing export command and its document-selection path. Done means the flag exports every document in Markdown for backup while preserving current export behavior, with the relevant CLI tests passing.

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