function isSnakecase( value ) {
	return ( isString( value ) && snakecase( value ) === value );
}
Tests if a value is a string in snake case.

@param {*} value - value to test
@returns {boolean} boolean indicating whether a value is a string in snake case

@example
var bool = isSnakecase( 'hello_world' );
// returns true

@example
var bool = isSnakecase( 'Hello World' );
// returns false

@example
var bool = isSnakecase( 'Hello_World' );
// returns false

@example
var bool = isSnakecase( '' );
// returns true

@example
var bool = isSnakecase( null );
// returns false