Show column toggles

This adds a showColumnToggles option to allow end users to control which columns are displayed in the table. If the option is true a button on the top right will reveal a dropdown with checkboxes for each column. To toggle a column off by default, add a hidden property to the column's field. If showColumnToggles is false, the hidden property will still hide the column.

The showColumnToggles option is false by default, so this release won't change anything until it is explicitly enabled.