feat: add censorChar option when using censored text

[tannerellen]

This adds the ability to specify the character used for the censor option called censorChar. An example would be...

   const input = blessed.textbox({
    inputOnFocus: true,
    censor: true,
    censorChar: "•",
  });

I chose to go with another option rather than have the censor property play double duty as it makes the data types clear.