add support for .vcpromptignore
i'd like to disable vcprompt in occasional large directories, such as kernel trees, since it's just takes too long to load the prompt.
my first thought is for vcprompt to check for well-known filename and just exit with "-" or some other shortish mark for "disabled in this directory".
i'm guessing this can also be achieved by having a similar shell-based mechanism for messing around with PS1. i think vcprompt way is probably cleaner, but thoughts welcome.
should be a simple patch either way.