Add 'today' to RelativeKeywords

[jroman00]

Carbon::setTestNow(Carbon::parse('2000-01-01 00:00:00'));

// 1999-12-31 00:00:00, Good!
$carbon = Carbon::parse('yesterday');
var_dump($carbon->toDateTimeString());

// 2016-03-16 00:00:00, Bad!
$carbon = Carbon::parse('today');
var_dump($carbon->toDateTimeString());

// 2000-01-01 00:00:00, Good!
$carbon = Carbon::parse('now');
var_dump($carbon->toDateTimeString());

// 2000-01-02 00:00:00, Good!
$carbon = Carbon::parse('tomorrow');
var_dump($carbon->toDateTimeString());

[codecov-io]

Current coverage is 99.29%

Merging #634 into master will not affect coverage as of 4d36d27

Powered by Codecov. Updated on successful CI builds.