Allow customizing the prompt

[jmmv]

Given that we have a concept of a current directory, the prompt should reflect that. But before overloading the current simple Ready prompt, I'd like to have a mechanism to customize it.

This is trivial by adding "special" environment variables, which we should have to customize various aspects from AUTOEXEC.BAS, but I'm not sure yet if it's wise to pollute the user's namespace with them.