QuestionTester: QuestionTester constructor
In MichielNoback/bintestr: The BioINformatics automated TESTing environment for R

This S3 class should be instantiated and passed to the Question constructor. It will be used to verify the users' answer and also provide feedback when a given answer is incorrect.

1	QuestionTester(tester_function, feedback_generator)

`feedback_generator`	this argument is used to rovide relevant feedback to the user. It can be either a simple `character` or a `function`. If it is a character, it is simply displayed when the answer is incorrect. If it is a function, it will be called when `tester` returns `FALSE`. It will be passed the user's incorrect answer and should print some appropriate feedback message.
`tester_fun`	the function that will be called to verify the question answer. It should only return `TRUE` or `FALSE`

## Instantiate a QuestionTester that tests for a character vector with a single
## "word" of length 5
## The feedback_generator generates some appropriate messages depending on the answer
t1 <- QuestionTester(
    function(x){is.character(x) && length(x)==1 && nchar(x)==5},
    function(x){
        if(! is.character(x)) "that is no word"
        else if(nchar(x) < 5) paste0(x, " has ", nchar(x), " characters and that is not 5")
    }
)
q1 <- Question("Please give a word of length 5", t1)