Add header function, axis labels, and tooltip rounding & font sizing

[isaac-florence]

Thank you for creating this excellent package, it's just what I've been looking for and more.

I have made some minor, non-breaking additions that I hope will be welcome.

Firstly, there is a new function to provide a title but using a different implementation to the current sg_title function. In order for this to be non-breaking I have called this sg_header. It returns a sg object rather than requiring shiny. It also has no additional imports so I hope is helpful.

Secondly, I have added a number of new parameters to the streamgraph function and JS. This includes rounding the figure shown by the tooltip (default is 10 d.p.), changing the tooltip font size, and adding axis labels.

Grateful for your thoughts