Support `rlang` dynamic dots

[mone27]

At the moment ggplot is using base ..., and rlang dynamic dots are not supported https://rlang.r-lib.org/reference/dyn-dots.html.

I think this could be a good addition to ggplot as it add the support of:

• splice arguments
• name injection
• ignoring trailing commands.
  At the same time it should not change the current features of ggplot.

The implantation should also be quite straightforward: replace list(...) with list2(...)

At the moment I can find that dynamic dots is supported only in guides.

If there is no reason in not having this feature I can try to make a PR for it.

[hadley]

This seems like a good idea to me. A PR would be great just to see how much code this would affect.