Adding a chainable property that will randomly return NULL

All formatters can be made optional by chaining `optional`.  When optional, the formatter will randomly return `NULL`, which can be useful for seeding non-required fields.  For example:

    $faker->optional->country
    
You can skew the randomization towards more nulls or less by passing an argument to `optional()`.  At 0, *only* `NULL` is returned.  At 1, it is never returned.

    $faker->optional(.75)->country