package.json: bump version 0.2.11 -> 0.2.12

New features:
- firenvim#focus_next and firenvim#focus_prev functions are now
  available in neovim in order to switch focus to the next/previous page
  element as <Tab> would in Firefox/Chrome (7f8c3c3)
- The firenvim add-on configuration page now points to instructions
  about configuring Firenvim (1014690)

Bug fixes:
- firenvim#eval_js should work better in firefox (1a56616)
- "tabs" permission has been removed - it turns out that it wasn't
  actually needed anymore (d316afe)