Add command-line option "-c|-console"

The new -c[onsole]:mode[:file] option allows to customize console
mode when starting gsl and optionally disable echo. Permissible
mode values:
    p   - print as requested
    d   - prefix with date and time (default)
    t   - prefix with time only
    dbg - datetime, with full flush to disk

If "file" is given, the output is captured to file.