Add external script. Fixes #957 #734 #1910

Includes a template and a simple script to reset file times before renaming.

This will probably require more instructions (possible on forum) on how to write new scripts (e.g how to retrieve, and return and modified file data from 'group'.

In settings, renamer, these scripts can be specified (in order to be run). this is an advanced setting.

<!---
@huboard:{"order":1982.0}
-->