code: function noLeadingSpaces(ast, file) {
    var lines = file.toString().split('\n');
    for (var i = 0; i < lines.length; i++) {
      var currentLine = lines[i];
      var lineIndex = i + 1;
      if (/^\s/.test(currentLine)) {
        file.message('Remove leading whitespace', {
          position: {
            start: { line: lineIndex, column: 1 },
            end: { line: lineIndex }
          }
        });
      }
    }
  }
docs: Lines that are just space characters are not present in
  the AST, which is why we loop through lines manually.