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 ?