Core twig filters are overwritten in siwapp Invoice printer

[joris]

A few (really usefull) twig core filters like date, format and nl2br are overwritten in siwapp's Common_Twig_Extension. This bugs me, because I wan't to be able to use the date and format functions from Twig not from Siwapp in my invoice template.

The list of Twig core filters can be found here:  http://twig.sensiolabs.org/doc/filters/index.html

Maybe it's a good idea to rename the filters that are added in the Common_Twig_Extension class. For backwards compatibility a flag can be set in the database for each existing invoice template to indicate that the old behaviour should be used. Another solution is to search and replace the filters in the invoice in the upgrade process.