CLI updates

[tbeseda]

Expand cli.mjs to enable standalone usage as a dependency.

Excerpt from updated README:

To use Enhance Styles in other applications or frameworks, install Enhance Styles from npm:

```shell
npm i -D @enhance/styles
```

The built in CLI, `enhance-styles`, takes two arguments: `--config=` and `--output=`. The `--config` argument is the path to your configuration file and the `--output` argument is the path where the .css file will be created. 

If you do not specify config, the default configuration will be used.  
You can use the default configuration or [create your own](#customize). We recommend a local project file like `./styleguide.json`.

If you do not specify output, the CSS will be printed to stdout.

```shell
npx enhance-styles --config=./styleguide.json --output=./public/enhance.css
```

[RMcNeely]

My big question here is what happens if I only want to change a few values in the config? Realistically I'm probably going to want to change something like typeScale and some theme colors but reading the cli.mjs file it looks like I need to copy the entire config.json from the root and make changes or I imagine it will break somewhere (even if it compiles fine).

Documentation wise I think what you have is good but that is something that I would address since its probably how this would be used as an external dependency.

[colepeters]

My big question here is what happens if I only want to change a few values in the config?

Great question! We need to test things out a bit more, but some of the fields used in the config have been set up with default values (see scales, for example), so that partial configs can be supplied and the defaults will fill in the rest. That said, we've only just started doing this, so there are likely some gaps still. Will definitely be looking at resolving this in near future releases!