Dynamically resize viewport based on content

[JordanMartinez]

Coming from FXMisc/RichTextFX#171 (and also FXMisc/RichTextFX#77)

It would be great if [the viewport] automatically changes it's height [and by implication, width] based on its content, so that [one] can set a minimum and a maximum and the box just gets bigger as additional content is added [or smaller as content is removed]. This could probably be done using minHeight and maxHeight.