Adding Varnish HTTP Cache input plugin (#1173)

* Adding Varnish HTTP Cache input plugin

* Applying PR feedback

- Linked to varnish in input README
- Updated README/CHANGELOG
- Cleaned up sampleConfig to remove formatting
- Shorted lines to 80 chars (except where test input requires long strings)
- Using internal.RunTimeout to wrap call to varnishtat
- Added dummy file for windows

plugins/inputs/varnish/varnish_windows.go

@@ -0,0 +1,3 @@
+// +build windows
+
+package varnish