urlselect: watch for new incoming URL when active

- when the bar is active, it will monitor current buffer for any new
  URL and add it to the end of list when it found one.

- changed the order of index so that oldest URL will have index 1 and
  the newest URL will have the last index

- changed the shortcut for running custom commands from just character
  into alt + character. this will allow you to type text while the bar
  is active

- readme: added screenshot + examples on binding command.

- main readme: remove the not working note for urlselect