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

[larmarange]

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