Pass dots to continuous_scale in scale_size/radius

[DarwinAwardWinner]

I tried to pass na.value to scale_size_continuous, and was surprised when it threw an error instead of passing it along to continuous_scale. This pull request fixes that by having it pass along ....

[has2k1]

Some refactoring of the code base a while ago got rid of ... from some of the user facing API. I think it was aiming for a more explicit documentation. So, if you are adding ..., you should add an item in the documentation of where those other parameters are defined.

[DarwinAwardWinner]

Would it be sufficient to add the following passage from other continuous scales (e.g. scale_alpha_continuous)?

 ...: Other arguments passed on to 'continuous_scale' or
      'discrete_scale' as appropriate, to control name, limits,
      breaks, labels and so forth.

[has2k1]

That should do.

[hadley]

I'd much rather you passed along specific arguments so that the document for individual scales remains self contained.

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