Add support for Vim's _ motion

From Vim's help files:

```
+               or                                      +
CTRL-M          or                                      CTRL-M <CR>
<CR>                    [count] lines downward, on the first non-blank
                        character linewise.

                                                        _
_  <underscore>         [count] - 1 lines downward, on the first non-blank
                        character linewise.
```

Since _ is + with a default of one less count, I added it to the same file as + and -. I couldn't directly implement it by taking away one, since a count of 0 is interpreted as no count given, so that case is handled separately.