Externalize the writing of content to a target

Introducing the target module in hugo.  This provides the simple
interface for writing content given a label (filename) and a io.Reader
containing the content to be written.

If site.Target is not set, it defaults back to the original behavior of
writing to file system.

In hugolib/site_url_test.go I have an InMemoryTarget for testing
purposes and use it to see if the final output of a render matches.

target/file.go

@@ -0,0 +1,9 @@
+package target
+
+import (
+	"io"
+)
+
+type Publisher interface {
+	Publish(string, io.Reader) error
+}