In some cases, a user of the falco engine may want to extend the falco
rules format to provide additional objects to the rules file.
To support that, add a new method set_rule_loader() that allows a user
to provide classes that derive from
rule_loader::{reader,collector,compiler} and read those additional
objects from the rules file.
Signed-off-by: Mark Stemm <mark.stemm@gmail.com>