If you are using 2.0.0-beta2, you can use tooltip callback and return array of strings there.

tooltips: {
    mode: 'single',
    callbacks: {
        afterBody: function(data) {
            var multistringText = ['first string'];
            // do some stuff
            multistringText.push('another string');

            return multistringText;
        }
    }
}

Actually all tool-tip callbacks support multiple lines of text, and you can use label callback as usual. It renders data label as tool-tip text by default.

Quoted from documentation:

All functions must return either a string or an array of strings. Arrays of strings are treated as multiple lines of text.

Example code:

tooltips: {
    callbacks: {
      label: (tooltipItem, data) => {
        if (tooltipItem.index % 2)
          return ['Item 1', 'Item 2', 'Item 3'];
        else
          return 'Single line';
      }
    }
  }

You can use tooltips footer callback,it will also not render coloured square for each list.

tooltips: {
  callbacks: {
    label: function(tooltipItem, data) {
      let label = data.datasets[tooltipItem.datasetIndex].label;
      let value = data.datasets[tooltipItem.datasetIndex].data[tooltipItem.index];
      return label + ': ' + value;
    },
    footer: function(tooltipItems, data) {
      return ['new line', 'another line'];
    }
  }
}