Now any value, not only strings, can be used through ${} notation

The ${} notation requres a string value. One cannot do something like:

    run 'counter = 0'
    find "This is visit no. ${counter}"

and should use something like this:


    run 'counter = 0'
    find "This is visit no. ${str(counter)}"

I find it unnecessary and annoying, so I am submitting a patch for a different behavior. The testes I've added (tests/test-dollar-sign-notation.*) shows the difference in behavior.