ui_plain: automatically lookup mimetype when mimetype-file is set

For sites that do not want to configure mime types by hand but
still want the correct mime type for 'plain' blobs, configuring
a mime type file is made possible. This is handy since such a
file is normally already provided (at least on Linux systems).

Also, this reflects the gitweb option '$mimetypes_file'

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>
Signed-off-by: Lars Hjemli <hjemli@gmail.com>

cgit.h

@@ -175,6 +175,7 @@ struct cgit_config {
 	char *index_info;
 	char *logo;
 	char *logo_link;
+	char *mimetype_file;
 	char *module_link;
 	char *project_list;
 	char *readme;