Bug 21576: Add a developer script to fix missing TT filters

Bug 21576: Add a developer script to fix missing TT filters

See bug 13618 and bug 21526.

We need a script to add missing filters, or replace wrong ones.

Test plan:
- Add unescaped variables to .tt files
- prove xt/find-missing-filters.t
will warn about them
- perl misc/devel/add_missing_filters.pl
will add the missing/wrong filters
- prove xt/find-missing-filters.t
will now be happy

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>