New command line option for custom Jack client name (sfizz_jack)

[lminiero]

Addresses #75. This adds a new -client_name property to choose a custom Jack client name, and the default if nothing is provided is sfizz, which keeps the previous behaviour:

-client_name (Jack client name); default: "sfizz";
-oversampling (Internal oversampling factor (value values are x1, x2, x4,
  x8)); default: "1x";
-preload_size (Preloaded value); default: 8192;

[paulfd]

Thanks!