Vendor all dependencies (Warning: Huge changeset.)

The vendor/ folder was created with the help of @FiloSottile's gvt and
vendorcheck.

Any dependencies of Caddy plugins outside this repo are not vendored.

We do not remove any unused, vendored packages because vendorcheck -u
only checks using the current build configuration; i.e. packages that
may be imported by files toggled by build tags of other systems.

CI tests have been updated to ignore the vendor/ folder. When Go 1.9 is
released, a few of the go commands should be revised to again use ./...
as it will ignore the vendor folder by default.

caddyhttp/httpserver/plugin.go

@@ -436,7 +436,7 @@ var directives = []string{
 	"root",
 	"index",
 	"bind",
-	"maxrequestbody", // TODO: 'limits'
+	"limits",
 	"timeouts",
 	"tls",
 
@@ -477,6 +477,7 @@ var directives = []string{
 	"pprof",
 	"expvar",
 	"push",
+	"datadog",    // github.com/payintech/caddy-datadog
 	"prometheus", // github.com/miekg/caddy-prometheus
 	"proxy",
 	"fastcgi",