Proposal: additive `configure`

* nbautoexport version: 0.2.1
* Python version: 3.9.1
* Operating System: Ubuntu 18.04.1

### Description

In the `clean` doc: https://nbautoexport.drivendata.org/cleaning/
I tried to add a folder to exclude, and it overwrote my old non-default `export_formats` config.
I think it would be more natural to not have everything overwritten.

### What I Did

```
❯ cat notebooks/.nbautoexport
{
"export_formats": [
"script",
"html"
],
"organize_by": "extension",
}
❯ nbautoexport configure notebooks/ \
--clean-exclude 'report/*'
Detected existing autoexport configuration at notebooks/.nbautoexport. If you wish to overwrite, use the --overwrite flag.
❯ nbautoexport configure notebooks/ --overwrite \
--clean-exclude 'report/*'
❯ cat notebooks/.nbautoexport
{
"export_formats": [
"script"
],
"organize_by": "extension",
"clean": {
"exclude": [
"report/*"
]
}
}
```