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