Support regex capture groups in replace command

See https://golang.org/pkg/regexp/syntax/ for the
supported syntax. Here are some examples:

```
replace "(foo)" "$1-bar"
replace "(foo)" "${1}-bar"
replace "(?P<group>foo)" "$group-bar"
replace "(?P<group>foo)" "$group-bar"
replace "(?P<key>\w+):\s+(?P<value>\w+)$" "$key=$value"
```

Closes #1115

internal/buffer/search.go

@@ -155,8 +155,12 @@ func (b *Buffer) ReplaceRegex(start, end Loc, search *regexp.Regexp, replace []b
 			l = util.SliceStart(l, end.X)
 		}
 		newText := search.ReplaceAllFunc(l, func(in []byte) []byte {
+			result := []byte{}
+			for _, submatches := range search.FindAllSubmatchIndex(in, -1) {
+				result = search.Expand(result, replace, in, submatches)
+			}
 			found++
-			return replace
+			return result
 		})
 
 		from := Loc{charpos, i}