Optional tooltip label

If multiple indicators of the same type are added, which is common for SMA, the tooltip displays all of them as the same (e.g. SMA). Allow an optional label parameter to be added to the indicator options to allow differentiation.

For example:
chart.addIndicator({
    id: 'APPL',
    type: 'sma', // will show default SMA in tooltip
    params: {
        period: 7
    }
});
chart.addIndicator({
    id: 'APPL',
    type: 'sma',
    label: 'sma 30', // will show SMA 30 in tooltip
    params: {
        period: 30
    }
});