Add a function equivalent to cyclocomp + quote

Using `cyclocomp` on expressions is a little clunky, since you always have to type `cyclocomp(quote(expr))`.

It would be convenient to have a `cyclocomp_q` function equivalent to this.  This should work, but not fully tested:

``` r
cyclocomp_q <- function(expr) {
  cyclocomp(substitute(expr))
}
```
