Allow for custom plugin "tasks" folder

[simlu]

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:

module.exports = require(`${__dirname}/../lib/sometask`);

Currently the tasks folder is hardcoded into loadNpmTasks.

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