Remove the word middleware (#1067)

* Rename middleware to plugin

first pass; mostly used 'sed', few spots where I manually changed
text.

This still builds a coredns binary.

* fmt error

* Rename AddMiddleware to AddPlugin

* Readd AddMiddleware to remain backwards compat

plugin/cache/README.md

@@ -0,0 +1,68 @@
+# cache
+
+*cache* enables a frontend cache. It will cache all records except zone transfers and metadata records.
+
+## Syntax
+
+~~~ txt
+cache [TTL] [ZONES...]
+~~~
+
+* **TTL** max TTL in seconds. If not specified, the maximum TTL will be used which is 3600 for
+    noerror responses and 1800 for denial of existence ones.
+    Setting a TTL of 300 *cache 300* would cache the record up to 300 seconds.
+* **ZONES** zones it should cache for. If empty, the zones from the configuration block are used.
+
+Each element in the cache is cached according to its TTL (with **TTL** as the max).
+For the negative cache, the SOA's MinTTL value is used. A cache can contain up to 10,000 items by
+default. A TTL of zero is not allowed.
+
+If you want more control:
+
+~~~ txt
+cache [TTL] [ZONES...] {
+    success CAPACITY [TTL]
+    denial CAPACITY [TTL]
+    prefetch AMOUNT [[DURATION] [PERCENTAGE%]]
+}
+~~~
+
+* **TTL**  and **ZONES** as above.
+* `success`, override the settings for caching successful responses, **CAPACITY** indicates the maximum
+  number of packets we cache before we start evicting (*randomly*). **TTL** overrides the cache maximum TTL.
+* `denial`, override the settings for caching denial of existence responses, **CAPACITY** indicates the maximum
+  number of packets we cache before we start evicting (LRU). **TTL** overrides the cache maximum TTL.
+  There is a third category (`error`) but those responses are never cached.
+* `prefetch`, will prefetch popular items when they are about to be expunged from the cache.
+  Popular means **AMOUNT** queries have been seen no gaps of **DURATION** or more between them.
+  **DURATION** defaults to 1m. Prefetching will happen when the TTL drops below **PERCENTAGE**,
+  which defaults to `10%`. Values should be in the range `[10%, 90%]`. Note the percent sign is
+  mandatory. **PERCENTAGE** is treated as an `int`.
+
+The minimum TTL allowed on resource records is 5 seconds.
+
+## Metrics
+
+If monitoring is enabled (via the *prometheus* directive) then the following metrics are exported:
+
+* coredns_cache_size{type} - Total elements in the cache by cache type.
+* coredns_cache_capacity{type} - Total capacity of the cache by cache type.
+* coredns_cache_hits_total{type} - Counter of cache hits by cache type.
+* coredns_cache_misses_total - Counter of cache misses.
+
+Cache types are either "denial" or "success".
+
+## Examples
+
+Enable caching for all zones, but cap everything to a TTL of 10 seconds:
+
+~~~
+cache 10
+~~~
+
+Proxy to Google Public DNS and only cache responses for example.org (or below).
+
+~~~
+proxy . 8.8.8.8:53
+cache example.org
+~~~