Add option to overwrite or extend default configs.

I love microbundle but sometimes we really would need a custom plugin or to overwrite a rollup config.
Therefore I would suggest the following:

new cli option:
```
--extendRollupConfig 'extend.js'
```

example extend file:
```
// extend.js (must return RollupConfig)
return {
plugins: [
myCoolPlugin() // Add new
terser({...}) // Overwrite terser plugin
]
outputOptions: {
systemNullSetters: true
}
}
```

The original config stays the same, but can be overridden if needed. Otherwise it is just extended.
I could take care of the implementation.

What do you think ?

Start by tracing the CLI option handling and default Rollup configuration, then inspect how the proposed extend.js module would be loaded. Done means the new option can extend or override the default configuration while preserving existing behavior.

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