notmuch config: Provide support for querying non-standard configuration values.

We might as well be general here, and allow the "notmuch config" command
to query any stored value from the configuration file, (whether or not
the rest of the code actually knows anything about that value).

notmuch-config.c

@@ -566,7 +566,7 @@ int
 notmuch_config_command (void *ctx, int argc, char *argv[])
 {
     notmuch_config_t *config;
-    const char *item;
+    char *item;
 
     if (argc != 2) {
 	fprintf (stderr, "Error: notmuch config requires two arguments.\n");
@@ -606,9 +606,36 @@ notmuch_config_command (void *ctx, int argc, char *argv[])
 	for (i = 0; i < length; i++)
 	    printf ("%s\n", tags[i]);
     } else {
-	fprintf (stderr, "Unknown configuration item: %s\n",
+	char **value;
-		 argv[1]);
+	size_t i, length;
-	return 1;
+	char *group, *period, *key;
+
+	group = item;
+
+	period = index (item, '.');
+	if (period == NULL || *(period+1) == '\0') {
+	    fprintf (stderr,
+		     "Invalid configuration name: %s\n"
+		     "(Should be of the form <section>.<item>)\n", item);
+	    return 1;
+	}
+
+	*period = '\0';
+	key = period + 1;
+
+	value = g_key_file_get_string_list (config->key_file,
+					    group, key,
+					    &length, NULL);
+	if (value == NULL) {
+	    fprintf (stderr, "Unknown configuration item: %s.%s\n",
+		     group, key);
+	    return 1;
+	}
+
+	for (i = 0; i < length; i++)
+	    printf ("%s\n", value[i]);
+
+	free (value);
     }
 
     notmuch_config_close (config);