Add link navigation and maybe even Org todo cycling

[PhilHudson]

Please consider adding one or both of the following subcommands:

• 'g' for 'link-hint-goto-link'
  This will move point to the first character of the link

• 't' for 'link-hint-cycle-todo-state-of-headline-containing-link' (org{-agenda}-mode only)
  This will move point to the first character of the link and invoke org{-agenda}-todo. This is the real reason I wanted 'g' above; I thought I should ask for it as a 'stretch' goal.

I suppose we could also do with 'e' for 'link-hint-edit-link', but that might be tough to generalize to all the different supported link types.

[noctuid]

#39 should at least be partially implemented to make this easier to do (e.g. link-hint-define-action to add default implementation).