Allow for custom plugin "tasks" folder

When creating grunt plugins, it it sometimes really awkward that we can't set a custom folder for the tasks.

Adding a dummy file `tasks/sometask.js` with the following content is pretty common when using other modules that rely on the `lib` or `src` path structure:
```javascript
module.exports = require(`${__dirname}/../lib/sometask`);
```

Currently the `tasks` folder is hardcoded into [loadNpmTasks](https://github.com/gruntjs/grunt/blob/master/lib/grunt/task.js#L370).

How would we best go about allowing customization here? An option parameter that can be passed into loadNpmTasks maybe?