Add option legend.inverseOrder to match tooltip.inverseOrder

[bf]

We have the option to reverse order of tooltips by setting tooltip.inverseOrder = true.
I suggest we create the same flag for the legends, so we can set the same order of labels both for the tooltips and the legend.

Therefore I propose legend.inverseOrder flag to be added.

Please let me know if you are open to this suggestion, I will then author a pull request for this great project.

[junedchhipa]

Yes, sounds reasonable.

If you want to create a PR, go ahead :)
My suggestion would be to add a CSS class when the legend.inverseOrder is true and use that CSS class to change the styles.

Refer #850

[junedchhipa]

Ignore my CSS suggestion above.
I have done the change.

[bf]

Perfect, thank you. Please don't forget to update the docs accordingly!

Also congratulations for this great project. It is really well thought out and a joy to use so far!