Implement 'workspace back_and_forth' (Patch by Michael Walle)

src/cfgparse.l

@@ -127,6 +127,7 @@ none                            { return TOK_NONE; }
 focus_follows_mouse             { return TOKFOCUSFOLLOWSMOUSE; }
 force_focus_wrapping            { return TOK_FORCE_FOCUS_WRAPPING; }
 force_xinerama                  { return TOK_FORCE_XINERAMA; }
+workspace_auto_back_and_forth   { return TOK_WORKSPACE_AUTO_BAF; }
 workspace_bar                   { return TOKWORKSPACEBAR; }
 popup_during_fullscreen         { return TOK_POPUP_DURING_FULLSCREEN; }
 ignore                          { return TOK_IGNORE; }