Simplify block arguments passing

This is a mess:

https://github.com/vladfaust/validations.cr/blob/028d4b48aecb37eb3e1c240a92aad0d68cd46de8/src/validations.cr#L175-L185

	{% if block.args[0] && block.args[0].stringify != "attr" %}
	{{block.args[0]}} = attr
	{% end %}

	{% if block.args[1] && block.args[1].stringify != "value" %}
	{{block.args[1]}} = value
	{% end %}

	{% if block.args[2] && block.args[2].stringify != "rule" %}
	{{block.args[2]}} = rule
	{% end %}

Provide feedback