* WIP: contact listing / searching.

this is the first part, adding a cache for all the contacts in mails
  (~/.mu/cache/contacts), which is a .ini-type file.

src/mu-runtime.h

@@ -83,6 +83,16 @@ const char* mu_runtime_xapian_dir      (void);
  */
 const char* mu_runtime_bookmarks_file  (void);
 
+/**
+ * get the mu contacts cache file name (typically,
+ * ~/.mu/contacts.cache); this can only be called after
+ * mu_runtime_init and before mu_runtime_uninit
+ * 
+ * @return the contacts cache file name as a string which should be not be
+ * modified, or NULL in case of error.
+ */
+const char* mu_runtime_contacts_cache_file  (void);
+
 
 /**
  * get the mu configuration options (ie., the parsed command line