Add additional validation rules

[JasonTheAdams]

The following rules would be useful:

RequiredIf

Conditionally sets a requirement based on the value of a sibling value

requiredIf:field,value

RequiredWith

Conditionally sets a requirement based on whether a sibling value is set and non-empty

requiredIf:fieldOne,fieldTwo

In

Limits a field's value to a fixed list

in:blue,yellow,red

DateTime

Checks that the value is a valid DateTime format and outputs a DateTime object. A specific format to validate for can be passed as an optional parameter.

datetime
datetime:yyyy-mm-dd