Unify the schema of configurable properties.

Currently dot-handy provides two ways to configure: commandline parameters and configuration files. However, they are connected loosely in a hard-coding way. The configuration files are flexible JSON files that can be written arbitrarily, and the overrides from the command line are bound to those properties through [hard-coding code in main.js](https://github.com/Automattic/dot-handy/blob/trunk/main.js#L17):

```
// TODO: this shouldn't be too hard to generalized to enable complete overriding of config flags through commandline params. I should consider to implement a config schema.
const parseOverrides = ( argv ) => {
const eligibleParams = [
'browser',
'cookies',
'currency',
'env',
'path',
'username',
'password',
'localstorage',
'locale',
];

const overrides = {};

for( const key of eligibleParams ) {
if ( argv[ key ] ) {
// FIXME: terrible!
if ( key == 'localstorage' || key == 'cookies' ) {
overrides[ key ] = JSON.parse( argv[ key] );
continue;
}
overrides[ key ] = argv[ key];
}
}

return overrides;
};
```

Ideally,

1. There should be a unified schema defining what's available in a configuration object.
2. How configuration properties bound to command line parameters won't need to be coded explicitly like this.

Start in main.js at parseOverrides and inspect how command-line arguments and configuration-file properties are currently connected. Define the configuration shape and replace the explicit eligibleParams mapping with a schema-driven connection; done means available properties and command-line overrides share one configuration definition without this hard-coded list.

Written by the indexing model from the issue text.