Show dot when there is no visible cursor

Disabled by default.

docs/API.md

@@ -53,6 +53,11 @@ protocol stream.
     should be sent whenever the container changes dimensions. Disabled
     by default.
 
+`showDotCursor`
+  - Is a `boolean` indicating whether a dot cursor should be shown
+    instead of a zero-sized or fully-transparent cursor if the server
+    sets such invisible cursor. Disabled by default.
+
 `capabilities` *Read only*
   - Is an `Object` indicating which optional extensions are available
     on the server. Some methods may only be called if the corresponding