optimize comment functionality

- cache comment column and line to increase performance
- toggle comments based on the current status of the line instead of either
  commenting or uncommenting all lines
- remove awkward functionality to increase/decrease of numbers within the
  comment string
  https://github.com/jceb/vim-commentary/blob/a3ae0df1d9e13941e9e8086637055841ce93f7bd/plugin/commentary.vim#L30