[vim keymap] Add count and empty query support for :s

If an empty query pattern is given for `:s[ubstitute]`, the previous query is used.

`[count]` is now supported. Per Vim documentation:

```
When [count] is given, replace in [count] lines,
			starting with the last line in [range].  When [range]
			is omitted start in the current line.
```