added header_separate function based on the suggestion in #541
diff --git a/NAMESPACE b/NAMESPACE
index 005da73..8f9df81 100644
--- a/NAMESPACE
+++ b/NAMESPACE
@@ -16,6 +16,7 @@
 export(footnote_marker_number)
 export(footnote_marker_symbol)
 export(group_rows)
+export(header_separate)
 export(html_dependency_bsTable)
 export(html_dependency_kePrint)
 export(html_dependency_lightable)