Thanks for adding a basic and useful function. Because it is so basic, we should document the edge cases: what happens if the arguments are `nil` or `number` is negative?
I think that it is also OK to explicitly document it as unspecified (which allows us to take shortcuts when converting YCP to Ruby later). Do you agree @jreidinger ?

Suggestion:

    @param input string to repeat (nil works as "")
    @param input number number of repetitions (>=0 or result is undefined)