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.