Support hline over a factor x range

Also, support multiple hlines (when `yintercept` is multiple or in the case of `facet`) properly.
By 'supporting properly', I mean with the proper number of traces, and corresponding/matching traces as found in the JSON (not just seen in the image).

Corresponding image diffs are found at https://github.com/ropensci/plotly/commit/fec1c5124d178d0a87151544e47110163b653c6d and https://github.com/ropensci/plotly/commit/c0b193deaf99b078b9a4bcfff022479022650b68

(Plot `cookbook-test-suite/bars-and-lines/basic-line-with-points...` is not related but I thought I would update it since it was broken.)

/cc @pedrodz