Add optional descriptions

[tharwan]

This PR adresses #86

[dokempf]

This is looking good. My only concern is that it currently only applies to those widgets that went through _construct_simple - missing e.g. all formats, object and array. But then again, similar things can be said about tooltips, I guess. If you have the capacity to extend the feature to these additional types, feel free to do so. If you do not have it, I am also fine with merging as is, as it is an improvement over the status quo.

[tharwan]

This is looking good. My only concern is that it currently only applies to those widgets that went through _construct_simple - missing e.g. all formats, object and array. But then again, similar things can be said about tooltips, I guess. If you have the capacity to extend the feature to these additional types, feel free to do so. If you do not have it, I am also fine with merging as is, as it is an improvement over the status quo.

I was not sure how to do it for the composed formats, but I also think it is most needed for the actual input fields which all go through the construct_simple.

[tharwan]

I would be very happy if we could merge this soon.

[dokempf]

@tharwan I implemented the missing types (array, object and datetime formats). With this change, I think I am ready to go.