middleware/hosts for /etc/hosts parsing (#695)

* add hosts middleware

* forgot pointer receiver

* add appropriately modified hostsfile tests from golang repo

* remove test artifacts, separate hostsfile parsing from caching and opening, remove unused metrics references, move middleware up the chain

* refactored the logic for creating records and filtering ip address versions. also got PTR lookups working

* Add README.md. Modify config to be more concise. Add zones list to config. Filter PTR responses based on zones list.

* add Fallthrough and return correct dns response code otherwise

* Simplified Hostsfile to only store hosts in the zones we care about, and by ip version. Added handler tests and improved other tests.

* oops, goimports loaded a package from a different repo

middleware/hosts/README.md

@@ -0,0 +1,45 @@
+# hosts
+
+*hosts* enables serving zone data from a /etc/hosts style file.
+
+The hosts middleware is useful for serving zones from a /etc/hosts file. It serves from a preloaded 
+file that exists on disk. It checks the file for changes and updates the zones accordingly. This
+middleware only supports A, AAAA, and PTR records. The hosts middleware can be used with readily
+available hosts files that block access to advertising servers.
+
+## Syntax
+
+~~~
+hosts FILE [ZONES...] {
+    fallthrough
+}
+~~~
+
+* **FILE** the hosts file to read and parse. If the path is relative the path from the *root*
+  directive will be prepended to it. Defaults to /etc/hosts if omitted
+* **ZONES** zones it should be authoritative for. If empty, the zones from the configuration block
+    are used. 
+* `fallthrough` If zone matches and no record can be generated, pass request to the next middleware.
+
+## Examples
+
+Load /etc/hosts file
+
+~~~
+hosts
+~~~
+
+Load example.hosts file
+
+~~~
+hosts example.hosts
+~~~
+
+Load example.hosts file and only serve example.org and example.net from it and fall through to the
+next middleware if query doesn't match
+
+~~~
+hosts example.hosts example.org example.net {
+    fallthrough
+}
+~~~