Improved implementation of underscore '_'

Fixes underscore for #681, #682, and #684

Currently internal normal mode cannot differentiate between c_ and d_. This change modifies the behavior of underscore internal mode to emulate c_, which removes the lines and leave a single blank row as opposed to leaving no blank row at all. This change is made because <count>d_ can be achieved using <count>dd instead, but not so for <count>c_.