Declare internal classes to represent a table elements

Following classes are added to make the easy to enhance in future.

* DataCell - table data
* Column - manage column attributes
* Row - manage cells

These are not exported.
So, the public interfaces and its results are not changed at all.

lib/index.js

@@ -12,21 +12,24 @@
         this.colmaxlen = [];
     };
     privates.ListItBuffer.prototype.nl = function() {
-        this.lines.push([]);
+        this.lines.push(new Row());
         return this;
     };
     privates.ListItBuffer.prototype.d = function(s) {
         if(this.lines.length <= 0) {
             this.nl();
         }
         var row = this.lines.length - 1;
-        var col = this.lines[row].length;
+        var col = this.lines[row].getCellLength();
         if(col >= this.colmaxlen.length) {
-            this.colmaxlen.push(s.length);
-        } else if(this.colmaxlen[col] < s.length) {
-            this.colmaxlen[col] = s.length;
+            var column = new Column();
+            this.colmaxlen.push(column);
+            col = this.colmaxlen.length - 1;
         }
-        this.lines[row].push(s);
+        this.colmaxlen[col].expandWidth(s.length);
+        var cell = new DataCell();
+        cell.setData(s);
+        this.lines[row].pushCell(cell);
         return this;
     };
     privates.ListItBuffer.prototype.toString = function() {
@@ -36,17 +39,65 @@
         var rows = [];
         this.lines.forEach(function(line) {
             var cols = [];
-            for(var col = 0; col < line.length; col++) {
-                var m = this.colmaxlen[col];
-                var s = line[col];
-                while(s.length < m) {
-                    s += ' ';
-                }
+            for(var col = 0; col < line.getCellLength(); col++) {
+                var s = this.colmaxlen[col].formatCell(line.getCell(col));
                 cols.push(s);
             }
             rows.push(cols.join(' '));
         }, this);
         return rows.join("\n");;
     };
     module.exports = exports;
+
+    //
+    // DataCell class
+    //
+    var DataCell = function() {
+        this.data = "";
+    };
+    DataCell.prototype.getData = function() {
+        return this.data;
+    };
+    DataCell.prototype.setData = function(data) {
+        this.data = data;
+    };
+
+    //
+    // Column class
+    //
+    var Column = function() {
+        this.width = 0;
+    };
+    Column.prototype.getWidth = function() {
+        return this.width;
+    };
+    Column.prototype.expandWidth = function(width) {
+        if(width > this.width) {
+            this.width = width;
+        }
+    };
+    Column.prototype.formatCell = function(cell) {
+        var m = this.getWidth();
+        var s = cell.getData();
+        while(s.length < m) {
+            s += ' ';
+        }
+        return s;
+    };
+
+    //
+    // Row class
+    //
+    var Row = function() {
+        this.cells = [];
+    };
+    Row.prototype.getCellLength = function() {
+        return this.cells.length;
+    };
+    Row.prototype.getCell= function(idx) {
+        return this.cells[idx];
+    };
+    Row.prototype.pushCell = function(cell) {
+        return this.cells.push(cell);
+    };
 }());