Support for Sinclair ZX Spectrum BASIC export.

Added a 3rd export option. The Third button exports an HTML file that contains ZX Spectrum BASIC source code. You can convert this code to TAP file and run it on an emulator or paste it in zxcoder.org.

At this point AGE generates only a BASIC text source file. I am thinking of directly generating TAP file ready to use on an emulator - or auto-launch the TAP file using JSSPeccy/QAOP.

The maze game exported can be played using keys WASD. The graphics are ASCII for now. 

I am thinking  to allow the use UDG decimal entries inside the image URLs textboxes so the exported BASIC game has UDG graphics.