[folding] support golang multiline strings

- VSCode Version: 1.10-insider
- OS Version: OS X 10.11.4

Steps to Reproduce:
1. Start a golang project.
2. Create a function in the form:

``` go
func foo() {
    bar := `
this is a multiline string.
`
}
```

Try and collapse `func foo() {` and the multiline string is not collapsed with it.
