add conf.int and exponentiate arguments to `tidy.cch()`

`tidy.cch()` is not consistent with other tidiers:

- it does not have a `conf.int` argument like other similar tidiers
- it does not have an `exponentiate` argument while such option would be relevant for such model as the exponentiate value of the coefficients are interpretable as Hazard Ratio, as displayed by the `summary()` method for these models