Hover effects!

**Is your feature request related to a problem? Please describe.**
We need links to change colour and underline on hover.

**Describe the solution you'd like**
Providing support for `:hover`, `:focus` and `:active` pseudo-classes by using PDF “actions for special effects” ([Adobe SDK](https://help.adobe.com/en_US/acrobat/acrobat_dc_sdk/2015/HTMLHelp/#t=Acro12_MasterBook%2FJS_Dev_ModifyUserInterface%2FUsing_actions_for_special_effects.htm)); other tutorials etc:

- [Inline Buttons: How to make a URL in a PDF Underline on Hover](http://documentgeek.blogspot.com/2009/05/inline-buttons.html)
- [Floating Text on PDF Documents](http://khkonsulting.com/2014/07/floating-text-pdf-documents/)
- [create a popup window when hovering over a word](https://answers.acrobatusers.com/create-a-popup-window-when-hovering-over-a-word-q119135.aspx)
- [Tutorial: Interactive PDF with rollover popups using Illustrator and Acrobat Pro](https://www.youtube.com/watch?v=HEyJuT8SSME)

I’m not sure how best to implement this in code, but it’s clear that it’s technically possible. Any pointers are welcome!