Generate placeholder psuedo-elements

_placeholder.sass

=input-placeholder
	&.placeholder
		@content
	&:-moz-placeholder
		@content
	&::-moz-placeholder
		@content
	&:-ms-input-placeholder
		@content
	&::-webkit-input-placeholder
		@content

// example usage
input
  +input-placeholder
	  color: red
	  
// generates
	input.placeholder
		color: red
	input:-moz-placeholder
		color: red
	input::-moz-placeholder
		color: red
	input:-ms-input-placeholder
		color: red
	input::-webkit-input-placeholder
		color: red