need to be able to modify column value before coersion

[iros]

For example, when parsing a dollar amount like $53,400 need to be able to first change to 53400.

Adding a property called before on column type override that's a function that takes a single value. For example:

var ds = new Miso.Dataset({
      data : {
        columns : [
          { name : 'vals', data : [1,2,3,4,5,6,7,8,9,10] }
        ]
      },
      columns : [
        { name : 'vals', type : 'number', before : function(v) {
            return v * 10;
        }}
      ],
      strict : true
    });

[iros]

Duplicate of #87??