Feature request: Generate docs.json, then exit

[runarfu]

I'm currently using (or rather misusing) elm-doc-preview to generate a docs.json for my application, using an elm-application.json as suggested in the README. I'm starting up elm-doc-preview for the sole purpose of generating the docs.json file, retrieving it with curl and then stopping the server again. This doesn't play well with build scripts, and I would like elm-doc-preview to have a CLI flag that does this:

• Generate docs.json and write it to file
• Exit with 0 if it succeeds or something else if the docs.json couldn't be generated.

[rlefevre]

You can use elm-doc-preview --output docs.json (or edp -o docs.json) in version 4.0.0 to generate the documentation and exit with a status code.