suggestion: added style

supports reverse highlighting using nu.config

```nushell
style: {
    text: white
    selected_text: {
        attr: r
    }
    description_text: white_dimmed
}
```

src/completion/base.rs

@@ -1,6 +1,7 @@
 use nu_ansi_term::Style;
 use std::ops::Range;
 
+
 /// A span of source code, with positions in bytes
 #[derive(Debug, Default, Clone, Copy, PartialEq, Eq, Ord, PartialOrd, Hash)]
 pub struct Span {