Allow configuring the editor colors

[jmmv]

While people can configure the colors of the console and make those changes relatively "persistent" via AUTOEXEC.BAS (not really, because the colors will be reset as soon as a CLEAR or RUN happens), the editor cannot be configured.

White on black is problematic for some people, and I've never liked a bunch of unmodifiable constants that exist in the code. Thus the colors should be configurable.

We need a mechanism to define configuration variables and then use those throughout the interpreter to replace hardcoded constants. An obvious way to do this is to define special variables like CONFIG_EDITOR_BG, set those via AUTOEXEC.BAS, and change the machine logic to treat them as special so that CLEAR does not wipe them out. We'd also need a CONFIG command that dumps them all (or a more generic VARS that prints all variables for introspection). But I'm not yet sure if that's the best design choice...