add current episode indicator in history widget

[regular]

highlight the episode that occupies the first pixel row (the current episode) in the "your path so far" widget.

• [x] css class to highlight an item in the history
• [x] find out which episode the reader is looking at currently
• [x] toggle the class on the li tags in the history
• [x] when an episode title is clicked in that widget, scroll its content to the top.