Missing report configuration defaultSummarizer

* **Version**: all
* **Platform**: all

There is a poorly-documented feature of the HTML Istanbul reporters (html, lcov, html-spa) called "defaultSummarizer". It would be nice if c8 could pass through this configuration option so it can be used.

I am willing to create a pull request for this issue myself.

Below is my documentation of what "defaultSummarizer" does from my [pull request to @web/test-runner](https://github.com/modernweb-dev/web/pull/2830/files#diff-614b8c6b865560ae3821293608d812c64cb4eb9ab846320eb508cd8ce8d89c1f).

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For some reporters, you can change the summary and hierarchy of reported files using the option `defaultSummarizer`.

- `pkg`
- The default setting
- Lists every subdirectory path on the main index page
- Shows the summary of the files in each subdirectory
- You must click into each subdirectory to see files
- `nested`:
- Displays nested file structure, like in your operating system or IDE
- Shows a deep summary of all the contents of each directory
- You must click through the directory structure to each file
- `flat`:
- All files are in one flat list
- No directory-based summaries
- There is only one page of results

**Example HTML report structures:**

_Actual file structure:_

- js
- a.js
- controller
- b.js
- manager
- factory
- c.js

_pkg_:

- js
- a.js
- js/controller
- b.js
- js/controller/manager/factory
- c.js

_nested_:

- a.js
- controller
- b.js
- manager
- factory
- c.js

_flat_:

- a.js
- controller/b.js
- controller/manager/factory/c.js

```