Add `reject` function to drop specified columns

[salbalkus]

This PR adds a reject function that drops the specified columns from the Table. It works by calling select on the columns of the table not specified by the user in the function call, thereby reusing all of the code that supports select.

I am adding this since I wrote it for my own work and figured it would be useful to other users. I'll admit that it might not be the cleanest solution -- open to better ways of doing this, or to changing the name of the function to something like dropcol if that name is more descriptive.