Add support for Pencil 2.

[cdoty]

Pencil 2 support currently works with this BIOS modification:
https://forums.atariage.com/topic/310073-coleco-pencil-2-modification/?do=findComment&comment=4608647

The BIOS modification works by enabling the SGM memory expansion to give the system at least 2k.

But, it should work with an unmodified BIOS if RAM is expanded to 2k. The system frequency also needs to be 50hz for the games to run at the right speed.

There is also a 16k RAM expansion that works with the Basic cartridge.