Pass legend customizations through own type_* functions

[grantmcdermott]

Closes #333
Closes #532

Quick notes:

• This was made much easier thanks to @vincentarelbundock's settings container. Just call settings$legend_args[[<arg>]] = <value> in the relevant data_<type> function.
• That said, I wasn't able to transfer all of the legend customization logic to the individual type_* constructor functions. We still need some "late" escape hatches for:
  • pt.cex for the "p" and "text" types; mostly to accommodate bubble plots. (The real issue is that we can't assign these values upfront in the case of dual legends. There might be a workaround, but it would require refactoring of the bubble / dual legend code.)
  • Off-color fills of "ridge" and "spineplot".
• I also went ahead and added some more context and guidance RE: re-using existing plot types in the custom types section in the Types vignette.