Add option to check uniqueness of values in array

[liuxh0]

Now that I am developing a plugin for Zowe CLI, I have the requirement that an array-typed option should have unique values. If a value appears twice, I would expect an error from Imperative. (For others maybe ignoring the repeated value makes sense.)

Currently I do not think I can achieve this using existing options. Do you think this requirement makes sense? If so, I would like to implement it. If not, we will just implement the logic in command handlers.