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/content_directory_test.go

@@ -0,0 +1,25 @@
+package source
+
+import (
+	"testing"
+)
+
+func TestIgnoreDotFiles(t *testing.T) {
+	tests := []struct {
+		path   string
+		ignore bool
+	}{
+		{"barfoo.md", false},
+		{"foobar/barfoo.md", false},
+		{"foobar/.barfoo.md", true},
+		{".barfoo.md", true},
+		{".md", true},
+		{"", true},
+	}
+
+	for _, test := range tests {
+		if ignored := ignoreDotFile(test.path); test.ignore != ignored {
+			t.Errorf("File not ignored.  Expected: %t, got: %t", test.ignore, ignored)
+		}
+	}
+}