Implement conversion for geom_abline

Strictly speaking, only the last two commits are about this conversion.  Feel free to check them out separately (https://github.com/ropensci/plotly/commit/0bab4d73e61e66150d25f4e586691a774065f405 and https://github.com/ropensci/plotly/commit/24cfe2a6f6dfd3823808f3e37728008c544895c5).

The rest is about clean-up.

/cc @pedrodz @xsaintmleux