document creation options

I'm using `codimd-cli import` to create a new document from Markdown. This is working on HackMD now (thanks!).

When importing, it looks like we need:

1. **The ability to specify which _team_ the document should be a member of.**

Right now, imported documents belong to the user. I cannot see how to import into a team.

1. **The ability to assign tags**

Regarding teams & organization, it appears the only way to organize documents _within a team_ is via tags. I think we can manage with tags for now; each Node.js working group could have its own tag. If something else is provided instead (e.g., folders), it's important that we _don't_ need to manage multiple logins or tokens.

1. **The ability to provide a custom slug (URL)**

We get a randomly-assigned URL after importing. This is a _nice-to-have_.

This stuff could also be part of an "editing" workflow, where we login, then import, then make another API call to move the document, assign tags, update the slug, etc.

Start at the `codimd-cli import` entry point and trace the existing document-creation API workflow. Determine how team membership, tags, and a custom slug could be supplied during import or through subsequent editing calls; done means the supported workflow is defined and documented for all requested options.

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