Add support for pattern validation

[fzaninotto]

user.creationView().fields([
    nga.field('name')
        .validation({ required: true, minlength: 3, maxlength: 100 }),
    nga.field('username')
        .attributes({ placeholder: 'No space allowed, 5 chars min' })
        .validation({ required: true, pattern: '[A-Za-z0-9\.\-_]{5,20}' }),
])