requireNewlineAfterArrayItems: impl

Fixes #317
```
var lastLine = 0;
els.forEach(function(element) {
  var curLine = element.loc.start.line;
  if (curLine === lastLine) {
    errors.add('Multiple elements at a single line in multiline array', {
      line: curLine,
      column: element.loc.start.column
    });
  }
  lastLine = curLine;
});
```