Fix exception when resizing both dimensions

This happened because buffers now resize themselves but they were
relying on Terminal to give them a blank line. The blank line was
coming back with the old columns value, causing an NPE.

Fixes #860

src/Interfaces.ts

@@ -48,7 +48,7 @@ export interface ITerminal {
   emit(event: string, data: any);
   reset(): void;
   showCursor(): void;
-  blankLine(cur?: boolean, isWrapped?: boolean);
+  blankLine(cur?: boolean, isWrapped?: boolean, cols?: number);
 }
 
 export interface IBuffer {