Add a “Go to Previous Position” feature

[aryanchoudharypro]

closes #115

[amirsol81]

@aryanchoudharypro Huge thanks for this! How many previous positions does it support? 10 or 20? And is it attached to ALT+Left and ALT+Right for moving among positions?

[trypsynth]

This doesn't work for me.

1. Open an epub with internal links.
2. Locate and press enter on one.
3. observe that your cursor moved to the position that the link points to.
4. Try to press alt+left arrow.

You hear no previous position, but I expect to be taken to where the link I just clicked was.