Added a columnTransform func to transform column names

[glenselle]

It's not uncommon for a CSV file to have strange or badly formatted column names. This PR adds a new transform function option (mapped to each column name) so column names can be overridden as needed. The default simply returns the existing value so nothing changes.

Usage might be something like this:

let parser = csv({
  columnTransform: function(name) {
    switch (name) {
      case 'some Weird Name':
        return 'new_name'
      default:
        return name
    }
  },
  columns: true
})