Loosen restrictions on window.open about:blank

Partial fix for #8768. brackets-shell fix to parse only `about:blank` URLs for sizing is pending.
@RaymondLim The goal is actually to allow extra stuff at the end -- see https://github.com/adobe/brackets-shell/pull/467#issuecomment-57551440.  Remember, this doesn't have to be a locked-down security thing -- the intent of this code is just to prevent naive/accidental misuse of `window.open()` by extension authors who don't understand the risks.