Check if numeric value is in enum

References [#381](https://github.com/erikhuda/thor/issues/381)

Validate if value is included in `enum` for numeric options. 

@sferik let me know if something is missing, wrong or you want to change some lines. I Added a test for string cases as well. Since this validation is the same for strings and numerics, it can be extracted into a private method, what do you think?

Thanks.