Feature Request: Filter List By Tag

It would be very useful to use the list command to return only notes that match a tag.

Use case: use hackmd-cli to export some notes and batch commit them to git. Not every note should be exported, but because `list` returns all notes, need to download them all and then process them based on tag in YAML metadata. Listing by tag would reduce the number of exports speeding up the process.

Start by tracing the `list` command in hackmd-cli and how it retrieves note metadata, then check how tags are represented in the YAML metadata. Done means the command can return only notes matching a requested tag while preserving existing list behavior; add or run the relevant command tests if the project provides them.

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