--format=json doesn't seem to do anything with full?

I am trying to use this - ironically - for Azure DevOps :) and it seems that i cannot export JSON from full coverage report.

with `gocover full`, I always get HTML output, even with `--format=json` (or `--format json`)

Start at the `gocover full` command entry point and trace how the `--format` option is handled. Reproduce the report with both `--format=json` and `--format json`; done means the full coverage report honors the JSON format instead of always producing HTML.

Written by the indexing model from the issue text.