valif: Validate and generate form input values

This class can be used to validate and output the values of form inputs.

It can check the values of a given list of inputs to verify whether the values satisfy several types of possible validation rules.

Currently it supports validation rules to verify whether the input value is sent and whether it is a valid e-mail address.

The class may also generate the value attributes of common and select input tags. Inputs with invalid values may be outputted with a different CSS style class.