Add new prop weekendColor to grid

[AquilesOliveiraDev]

Hi, i would like to thank for this lib.

In our project, we have some rules applied on weekends and we miss being able to apply colors in this column, so we would like to contribute to this feature.

[Dozalex]

@AquilesOliveiraDev Thanks for the PR. I supported it in my fork https://github.com/Dozalex/gantt-task-react