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.cfg

@@ -34,13 +34,14 @@
 120:loadbalance:loadbalance
 130:dnssec:dnssec
 140:reverse:reverse
-150:kubernetes:kubernetes
-160:file:file
-170:auto:auto
-180:secondary:secondary
-190:etcd:etcd
-200:proxy:proxy
-210:whoami:whoami
-220:erratic:erratic
+150:hosts:hosts
+160:kubernetes:kubernetes
+170:file:file
+180:auto:auto
+190:secondary:secondary
+200:etcd:etcd
+210:proxy:proxy
+220:whoami:whoami
+230:erratic:erratic
 500:startup:github.com/mholt/caddy/startupshutdown
 510:shutdown:github.com/mholt/caddy/startupshutdown