Add data files support in themes

If duplicate keys, the main data dir wins.

Fixes #892

helpers/general.go

@@ -19,13 +19,13 @@ import (
 	"encoding/hex"
 	"errors"
 	"fmt"
+	bp "github.com/spf13/hugo/bufferpool"
+	"github.com/spf13/viper"
 	"io"
 	"net"
 	"path/filepath"
 	"reflect"
 	"strings"
-
-	bp "github.com/spf13/hugo/bufferpool"
 )
 
 // Filepath separator defined by os.Separator.
@@ -100,6 +100,10 @@ func BytesToReader(in []byte) io.Reader {
 	return bytes.NewReader(in)
 }
 
+func ThemeSet() bool {
+	return viper.GetString("theme") != ""
+}
+
 // SliceToLower goes through the source slice and lowers all values.
 func SliceToLower(s []string) []string {
 	if s == nil {