This PR adds a few options to the Integer assert:

• allowString: This option allows the assert to validate integers represented as strings.
• unsigned: This option forces only simple unsigned integers.

The question here is if we want to allow integer assert to be a string and the impact for users confronted with an integer assert that forces the input to be a string. Also it would be nice to come to a conclusion on distinguishing between integers with leading zeros or not and if an option should be presented for that matter.