Add raw modeline support.

This allows specifying a modeline in the config for the 'mode' key
in the output section, such as one you would get from cvt.
diff --git a/weston.ini b/weston.ini
index 375f4ee..c2d6369 100644
--- a/weston.ini
+++ b/weston.ini
@@ -36,8 +36,8 @@
 
 #[output]
 #name=LVDS1
-#mode=off
+#mode=1680x1050
 
 #[output]
 #name=VGA1
-#mode=1280x1024
+#mode=173.00  1920 2048 2248 2576  1080 1083 1088 1120 -hsync +vsync