Address Early Adpoter's feedback

This PR addresses all of the points in the feedback as discussed in #8, for @honzajavorek to review & merge
Hm, as I dive more into programming, I realize this `value` vs. `value type` is really quite a mess. What comes to my mind (and it's also like that in JSON Schema) - couldn't we rename `Value Type` to `Item Type`? The word item isn't used anywhere else and in my opinion it unambiguously refers to "array item", "list item", something like that.