Parse yargs numbers to strings

Add conversion to strings of Yargs arguments which are numbers. All current commands expect string values as arguments, so we do not need numbers as they are causing exceptions in some cases. For example if you use appbuilder simulate --path 9, where 9 is valid directory, you will receive an exception.
Merge pull request #72 from telerik/vladimirov/validate-number-parameters

Add validation for number params