Allow append skipped rules (#346)

* Allow appending to skipped rules

If a rule has an append attribute but the original rule was skipped (due
to having lower priority than the configured priority), silently skip
the appending rule instead of returning an error.

* Unit test for appending to skipped rules

Unit test verifies fix for appending to skipped rules. One rules file
defines a rule with priority WARNING, a second rules file appends to
that rules file, and the configured priority is ERROR.

Ensures that falco rules without errors.

test/rules/append_single_rule.yaml

@@ -0,0 +1,3 @@
+- rule: open_from_cat
+  append: true
+  condition: and fd.name=/tmp