add `0` key for beginning-of-line

This adds a shortcut for the `0` key to take the user to the beginning
of a given line, like Vim. It simply calls `line_home` like `^` does,
but this one is in my muscle memory from Vim usage.

By the way, thanks for developing this. I was looking for a dead-simple
command-line CSV viewer, and this fits the bill.