Provide a live (iFrame) preview of various flag texts in action

In the Fivestar module, there is an excellent live preview of all the Fivestar settings as you adjust the various options (number of stars, various flag texts, Fivestar format, etc.) This helped tremendously with users understanding what various options did in the UI. Flag has already got a lot of textfields, and there are a lot of issues suggesting more yet:

#301680: Create message: "Login to [flagname] [nodetitle]"
#1138162: Raw tokens shown in UI for bulk operations and empty bookmarks page

Plus some issues that would increase the flexibility/complexity of the Flag text itself:
#1256012: Allow html text instead of plain link text

To prevent Flag from becoming entirely confusing with its dozen of textfields, we should imitate Fivestar and provide a live preview of all of these options as the user populates the fields.

Examples of things to include in the Preview:

Flag link:
Bookmark

Unflag link:
Remove from Bookmarks

Anonymous message:
Login or Register to boomark this content

Bulk operations:

- Choose action -
Bookmark
Unbookmark

We might also figure out a way to do things like display the AJAX behavior as needed. But just getting the basic behavior in place would be a huge help.