Add ability to split configuration files using magic markers

### BEGIN-MYMARKER-MAGIC
# part active for MYMARKER in file_MYMARKER.ext
### END-MYMARKER-MAGIC
### BEGIN-OTHERMARKER-MAGIC
# part active to OTHERMARKER in file_OTHERMARKER.ext
### END-OTHERMARKER-MAGIC

Signed-off-by: Sébastien Gross <seb•ɑƬ•chezwam•ɖɵʈ•org>