Customize `less` instance

Would be great if I could pass in a specific less instance to control the instance used here:

https://github.com/egoist/rollup-plugin-postcss/blob/71593d9f4698ce564482b86cc2fa7f69626e8b8a/src/less-loader.js#L15

Currently, it's loaded relatively to the file with the plugin module, not the project, and in a monorepo with node_modules hoisting this can use a different version than anticipated.

```js
import less from 'less';

...
postcss({
use: {
less: {
less,
}
}
})
```