Allow renderValue/getValue to take a key

[hailwood]

Right now if we want a custom cell to use multiple attributes we need to pull them off row.original E.g.

builder.accessor('name', {
  header: 'Name',
  enableSorting: true,
  cell: ({renderValue, row: {original: {user: {email}}}}) => (
    <div>
      <span>{renderValue()}</span>
      <span>{email}</span>
    </div>
  ),
}),

It would be cool if instead renderValue or getValue would by default use the current cell accessor, but could also be given a data key and would render that instead:

builder.accessor('name', {
  header: 'Name',
  enableSorting: true,
  cell: ({renderValue}) => (
    <div>
      <span>{renderValue()}</span>
      <span>{renderValue('user.email')}</span>
    </div>
  ),
}),