Add configuration options to control terminal sizes (#475)

This allows for configuring the size of computers and pocket computers, 
as well as the max size of monitors.

There's several limitations with the current implementation, but it's
still "good enough" for an initial release:
 - Turtles cannot be resized.
 - GUIs do not scale themselves, so "large" sizes will not render within
   the default resolution.

src/main/java/dan200/computercraft/shared/turtle/blocks/TileTurtle.java

@@ -87,7 +87,7 @@ public class TileTurtle extends TileComputerBase implements ITurtleTile, Default
     {
         ServerComputer computer = new ServerComputer(
             getWorld(), id, label, instanceID, getFamily(),
-            ComputerCraft.terminalWidth_turtle, ComputerCraft.terminalHeight_turtle
+            ComputerCraft.turtleTermWidth, ComputerCraft.turtleTermHeight
         );
         computer.setPosition( getPos() );
         computer.addAPI( new TurtleAPI( computer.getAPIEnvironment(), getAccess() ) );