ui-plain: add enable-html-serving flag

Unrestricts plain/ to contents likely to be executed by browser.
diff --git a/cgit.h b/cgit.h
index 4b4bcf4..de5c94a 100644
--- a/cgit.h
+++ b/cgit.h
@@ -101,6 +101,7 @@
 	int enable_log_linecount;
 	int enable_remote_branches;
 	int enable_subject_links;
+	int enable_html_serving;
 	int max_stats;
 	int branch_sort;
 	int commit_sort;
@@ -235,6 +236,7 @@
 	int enable_log_linecount;
 	int enable_remote_branches;
 	int enable_subject_links;
+	int enable_html_serving;
 	int enable_tree_linenumbers;
 	int enable_git_config;
 	int local_time;