Request: Simple validation function

[aensley]

Thank you for this useful library. It's really great. I have a small feature request. Could you add a validateCronExpression() or isValidCronExpression() static function to CronExpression?

The reason I need this is that there will be situations where schedules are created programatically, and I want to make sure an invalid schedule isn't saved for later usage, causing errors on every run.

Right now, I'm doing this to accomplish what I need:

try {
    \Cron\CronExpression::factory($cronExpression);
} catch (\InvalidArgumentException $e) {
    LogUtil::error('Invalid cron expression "' . $cronExpression . '"');
    return false;
}

// $cronExpression is valid. Proceed.

It would be nice if I could do this:

if (!\Cron\CronExpression::isValidCronExpression($cronExpression)) {
    LogUtil::error('Invalid cron expression "' . $cronExpression . '"');
    return false;
} else {
    // $cronExpression is valid. Proceed.
}

It's a very small change as far as the code I write, but I feel like my current solution is a bit ugly and hack-ish since the underlying code might change in how invalid expressions are handled.

[mtdowling]

Sounds good to me.

[aensley]

Closing since #87 was merged.