Replace underscores in variable names when using interact

If your variable names in interact have underscores in them, then the widgets will be displayed with labels like "my_variable:", which is less desirable than "My variable:" or even "my variable:"

This pull request replaces underscores with spaces, and also capitalizes the first letter, but only if there are no other capitals. This is because people usually code using lower-case variable names, so the idea is that if there is already a capital letter, it is probably there for a reason.