Assert with optinal description #7

[integer]

• new feature Assert with custom message. #7
• BC break? no

With this PR you can add description to your asserts and this description will be printed in error messages.

$schema = Expect::string()
        ->assert('ctype_digit', 'Is number')
        ->assert(function ($s) { return strlen($s) >= 3; }, 'Minimal lenght');

[dg]

It could be implemented in an alternative way, but I don't know what's better.

$schema = Expect::arrayOf('string')
	->assert(function ($v) { 
		if (count($v) % 2 !== 0) { // count must be even number
			throw new AssertException('Even items in array');
		}
	}); 

[mabar]

I would like exception, it allows non-statical error messages

[dg]

After experience with forms and addRule() I would prefer exceptions today.

[integer]

Exceptions requires more writing, but you can prepare better message. I will try exceptions in my project. Thank you for hint.