Add themesDir option to configuration

themesDir can be used to change the default
path of the themes folder.

Fixes 1556

content/overview/configuration.md

@@ -130,7 +130,8 @@ Following is a list of Hugo-defined variables that you can configure and their c
     staticdir:                  "static"
     # display memory and timing of different steps of the program
     stepAnalysis:               false
-    # theme to use (located in /themes/THEMENAME/)
+    # theme to use (located by default in /themes/THEMENAME/)
+    themesdir = "themes"
     theme:                      ""
     title:                      ""
     # if true, use /filename.html instead of /filename/