Added a bunch of new post-question validation hooks and a new pre-hook

Hello again, I've been hard at work today :smile: 

This time, I'm including a bunch of new hooks which should cover almost every use case out there...

The post-question hooks added are:

* **to_integer**: Converts an answer to an integer
* **to_decimal**: Converts an answer to a float
* **validate_choices**: Validates an answer in a given set of choices
* **validate_regex**: Validates an answer against a given regex
* **validate_datetime**: Validates an answer against a chosen datetime format

And the pre-question hook added is:

* **set_current_datetime**: Set the default answer of a question to the current date and time

Test cases provided here give 100% coverage of the new hooks :smile: 

Cheers
Fotis