add an addColumn method

[iros]

// adds empty column
dataset.addColumn("name");

// adds a column with data
dataset.addColumn("name", [1,2,3,4,5]);

// another way to add data to a column
dataset.addColumn("name").each(function(row) {
  row.name = 12;
});

Should return reference to dataset so we can further chain awesomeness.

[rwaldron]

What about allowing for default "fill"?

dataset.addColumn("name", { def: "" });

or...

dataset.addColumn({ col: "name", def: "" });

Would fill all rows under "name" with ""

Just a thought.