feat(lib/policy/expressions): add system load average to bot expression inputs

This lets Anubis dynamically react to system load in order to
increase and decrease the required level of scrutiny. High load? More
scrutiny required. Low load? Less scrutiny required.

data/botPolicies.yaml

@@ -74,6 +74,21 @@ bots:
     weight:
       adjust: 10
 
+  ## System load based checks.
+  # If the system is under high load, add weight.
+  - name: high-load-average
+    action: WEIGH
+    expression: load_1m >= 10.0 # make sure to end the load comparison in a .0
+    weight:
+      adjust: 20
+
+  # If it is not, remove weight.
+  - name: low-load-average
+    action: WEIGH
+    expression: load_15m <= 4.0 # make sure to end the load comparison in a .0
+    weight:
+      adjust: -10
+
   # Generic catchall rule
   - name: generic-browser
     user_agent_regex: >-