Options object for panto.
const PantoOptions = require('panto-options');
const options = new PantoOptions({cwd: '.'});
options.extend({cwd: 'cwd', list: 'all'});
options.set('current', 'sub');
options.get('list')// 'all'
options.get('absense') // undefined
options.get('absense', 'foo') // 'foo'
options.get() // {cwd: 'cwd', list: 'all', current: 'sub'}
options.rm('cwd') //true