Introduce source.Filesystem

This provides an abstraction over how files are processed by Hugo.  This
allows for alternatives like CMS systems or Dropbox, etc.

source/filesystem_test.go

@@ -0,0 +1,32 @@
+package source
+
+import (
+	"bytes"
+	"testing"
+)
+
+func TestEmptySourceFilesystem(t *testing.T) {
+	src := new(Filesystem)
+	if len(src.Files()) != 0 {
+		t.Errorf("new filesystem should contain 0 files.")
+	}
+}
+
+func TestAddFile(t *testing.T) {
+	src := new(Filesystem)
+	src.add("foobar", bytes.NewReader([]byte("aaa")))
+	if len(src.Files()) != 1 {
+		t.Errorf("Files() should return 1 file")
+	}
+
+	f := src.Files()[0]
+	if f.Name != "foobar" {
+		t.Errorf("File name should be 'foobar', got: %s", f.Name)
+	}
+
+	b := new(bytes.Buffer)
+	b.ReadFrom(f.Contents)
+	if b.String() != "aaa" {
+		t.Errorf("File contents should be 'aaa', got: %s", b.String())
+	}
+}