Problem with multiline search

`test.txt`
```
a line

// bellow is a line
a line

a line
```

---

`:CtrlSF 'a line'` works fine:

**4** matched lines across **1** files. Done!

`a line`

// bellow is `a line`
`a line`

`a line`

---

However `:CtrlSF -R '//.*a line'.*\n.*a line.*` prints file with match, but doesn't count it nor highlight:

**0** matched lines across **1** files. Done!

a line

// bellow is a line
a line

a line

---

Also, rg `--multiline` flag is not supported. Tho I was able to test it with `let g:ctrlsf_extra_backend_args={ 'rg': '--multiline' }`, it worked the same way as simulated above.

---

| field | value |
|:-------:|:------------------------------------:|
| os | *Ubuntu 20.04* |
| vim | *Vim 8.2* |
| backend | *ag 2.2.0* |
| locale | *en_US.UTF-8* |

- vimrc:

```vim
let g:ctrlsf_ackprg = 'ag'
```