Add middleware/erratic (#471)

This middleware allows playing with responses. Only one type is
implemented: it allows you to drop queries. I.e. withhold the response
from the client.

middleware/erratic/README.md

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+# erratic
+
+*erratic* is a middleware useful for testing client behavior. It returns a static response to all
+queries, but the responses can be delayed by a random amount of time or dropped all together, i.e.
+no answer at all.
+
+~~~ txt
+._<transport>.qname. 0 IN SRV 0 0 <port> .
+~~~
+
+The *erratic* middleware will respond to every A or AAAA query. For any other type it will return
+a SERVFAIL response. The reply for A will return 192.0.2.53 (see RFC 5737), for AAAA it returns
+2001:DB8::53 (see RFC 3849).
+
+## Syntax
+
+~~~ txt
+erratic {
+    drop AMOUNT
+}
+~~~
+
+* **AMOUNT** drop 1 per **AMOUNT** of the queries, the default is 2.
+
+## Examples
+
+~~~ txt
+.:53 {
+    erratic {
+        drop 3
+    }
+}
+~~~
+
+Or even shorter if the defaults suits you:
+
+~~~ txt
+. {
+    erratic
+}
+~~~
+
+## Bugs
+
+Delaying answers is not implemented.