add r() and w() to select_vars

These functions are less explicit than the current one allowed in `select_vars`, but quicker to type.
`w()` allows to use wildcards. 
`r()` is another name for the function matches.

```R
select_vars(c("id", "id2", "other"), w("id*"))
   id   id2 
 "id" "id2"
select_vars(c("id", "id2", "other"), r("^id"))
   id   id2 
 "id" "id2"
```