Add grid customization

[jcobb88]

Currently the grid can be very/too strong, especially when you change the background color. A customization method in ChartBuilder would be helpful.

[matoos32]

Thanks again for the input. Will have a look when I'm able to.

[matoos32]

Hello @jcobb88, I added configuring the major and minor gridlines as of pull request #65.

Hope this helps!