install: Add support for sets, e.g. HenrikBengtsson/{listenv,globals}

The following two should be equivalent

``` r
source('http://callr.org/install#listenv,globals')
source('http://callr.org/install#{listenv,globals}')
```

as well as these two

``` r
source('http://callr.org/install#HenrikBengtsson/listenv,HenrikBengtsson/globals')
source('http://callr.org/install#HenrikBengtsson/{listenv,globals}')
```
