Create more independent item validations

Additional validators that are needed for independent values. Dependent value valdiators are #7.

For IComparable:

• [ ] GreaterThan
• [ ] GreaterThanOrEqual
• [ ] LessThan
• [ ] LessThanOrEqual
• [ ] Required

Dates:

• [ ] RelativeToNow One syntax would be named values for day, hour, etc.

Strings:

• [ ] MinLength
• [ ] MaxLength
• [ ] MinMaxLength

If you would like to work on these, please use the GreaterThanValidator.cs as a model, and include tests in ValidatorTests.cs where you can parallel the GreaterThanValidator tests, which you are also welcome to improve. If you create a sub-issue or comment here that you are working on one of them, it will help avoid double work.

Please propose more!