Add --bell and --bwatch command line options

Add `--bell` and `--bwatch` (shortcut for `--bell --watch`) command line options.  --bell rings a system bell when a file is compiled.  For my use case, this is really nice because I typically have Visual Studio in one window and my browser in another, and I don't want to give up any screen real estate to the cmd.exe to keep an eye on my `sass --watch`.  With the custom framework we use, it can take up to 5 seconds sometimes to fully compile everything, and having the bell has really made my life easier in that regard.