Add ability to open links in new tabs using long right-click

[Infocatcher]

Requested here: https://addons.mozilla.org/addon/right-links/reviews/497914/

Something like

Long right-click
└─ Options for long right-click
Long left-click
└─ Options for long left-click
Options
├─ [ ] Enable right-click
├─ [v] Enable long right-click
└─ [v] Enable long left-click