Add `isDirty` command

[Amitai Burstein]

Removed this part from #2 , and added a proper issue to it :)

Maybe we should add something like this?:

isDirtyWith : Editable record -> (record -> record -> Bool) -> Bool
isDirtyWith editableRecord compareFunc =
    case editableRecord of
        ReadOnly _ ->
            False

        Editable originalRecord updatedRecord ->
            compareFunc originalRecord originalRecord


isDirty : Editable record -> Bool
isDirty editableRecord =
    isDirtyWith editableRecord (==)

[Christoph Hermann]

@amitaibu I like this!