Add example for Display7

[peppergrayxyz]

The 7 segment display (display7) is a quite useful feature, but it is nowhere explained how to use it from verilog.
(Feature was added here: tilk/digitaljs#84)

The only place, where u can find how to do it is tilk/yosys2digitaljs:1296

if (dev.type == 'Output') {
    if (dev.bits == 1)
        dev.type = 'Lamp';
    else if (dev.bits == 8 && (dev.label == 'display7' || dev.label.startsWith('display7_')))
        dev.type = 'Display7';
    else
        dev.type = 'NumDisplay';

It would be great to:

1. Have an example in the UI ("Load Example Code")

module display_7(
  output [7:0] display7_d1
);
  assign display7_d1 = 8'b11111111;	
endmodule

Probably also good to add the example to the test cases for yosys2digitaljs

2. Have the ability to switch any 8bit output to display7

(I couldn't find the code that is responsible for the drop down)

Happy to create pull requests!