Improved terminal cursor style

* Changed "dim" cursor visibility from invisible to almost transparent
  and in the same color as default background. This improves visibility
  of cursor, if we are usign application (like vim for example) with
  dark background and the "Black on White" user CSS is set.

shellinabox/styles.css

@@ -67,7 +67,10 @@
 }
 
 #vt100 #cursor.dim {
-  visibility:           hidden;
+  background-color:     white;
+  opacity:              0.2;
+  -moz-opacity:         0.2;
+  filter:               alpha(opacity=20);
 }
 
 #vt100 #cursor.inactive {

shellinabox/white-on-black.css

@@ -3,6 +3,13 @@
   color:            black;
 }
 
+#vt100 #cursor.dim {
+  background-color: black;
+  opacity:          0.2;
+  -moz-opacity:     0.2;
+  filter:           alpha(opacity=20);
+}
+
 #vt100 #scrollable {
   color:            #ffffff;
   background-color: #000000;