More inputs in coords 

Follow-up of #67.

`ci.coords` should be able to accept every coordinate that `coords` takes.

An application would be to add error bars on PR curves

```
    r <- roc(aSAH$outcome, aSAH$s100b)
    coords(r, x = seq(0, 1, .1), input="recall", ret="precision")
    ci <- ci.coords(r, x = seq(0, 1, .1), input = "recall", ret="precision")
    plot(ci)
```