[features-] add command to hide repetitive columns

[midichef]

This is a command to hide any column that contains one value repeated in every row. Is this worth having? It's often the first command I run when opening large tables with such uninformative columns.

I'm not sure what to call the command. Right now it is hide-unvarying-cols, but perhaps hide-uniform-cols is better? hide-repetitive-cols is too hard to spell.