Allow `use_virtualenv()` to fall back to looking for system wide installations. 

For example

```r
use_virtualenv("r-keras")
```
Would look for:

- `~/.virtualenvs/r-keras`
- `/opt/virtualenvs/r-keras`

By default assembled from:
```r
c(Sys.getenv("WORKON_HOME", "~/.virtualenvs/"),
  Sys.getenv("RETICULATE_VIRTUALENV_SEARCH_PATH", "/opt/virtualenvs/"))