Add ggplot2 scales extensions

They wouldn't do much, just change the default order of the sequence based on the era's `direction` attribute, and add abbreviated labels somewhere.

It would also be nice to support paired eras when plotting (#17).

Resources:

* [Extending ggplot2 vignette](https://cran.r-project.org/web/packages/ggplot2/vignettes/extending-ggplot2.html), "Doing more with facets"

Read the linked “Extending ggplot2” vignette, especially “Doing more with facets,” then inspect the package’s existing plotting entry points. Clarify how era direction should order sequences, where abbreviated labels belong, and how paired eras should plot; done means these behaviors are specified and covered for the relevant plots.

Written by the indexing model from the issue text.