Adding ability to provide a 'size' attribute on the modal cmp.

[mbowen000]

Hey Strike Team!

Not sure if I'm missing anything big for testing this but it's a simple change. I wanted to have support for large modals in my project (without overriding CSS), so I created this simple change. I'm not a big OSS contributor so hopefully I'm not missing anything big here!

• Supply a "size" attribute (optional). Right now the only supported SLDS modal size variant is "large"
• This will add the slds-modal_large class onto the div.slds-modal element

I only tested that it works with it on and off. I did not see anything else that would be impacted by this feature, but let me know if you would like to see anything else! Thanks!