FIX: shift+click on links

Default browser's behavior when shift+clicking was messed up.

This adds the shift key modifier to the list of click modifiers in both
`click_track` and `discourse`.

Also updated & refactored a bit the `click_track_spec`.
Merge pull request #796 from ZogStriP/fix-shift-clicks-on-links

FIX: shift+click on links