adds Config option to disable bell; DisableBell

[henderjon]

In my app, I'd like to disable the bell programmatically. It seems that making that a config option makes sense and someone else might also want to do so.

The whitespace changes are a result of gofmt.

[wader]

Hi! there is a bit more active fork at https://github.com/ergochat/readline that you might be interested in