Support geom_vline() conversion

* Also enhanced it for geom_hline() at the same time: You can now pass a vector of values to the intercept parameter, to draw multiple lines (horizontal or vertical).

* Image diffs can be seen at https://github.com/ropensci/plotly/commit/08a4199f13af9c7cdd0c32cae925ffef9494bce0
- New images are `tests/testthat/test-ggplot-vline-*.png`, `tests/testthat/test-ggplot-vline-multiple-*.png`, `tests/testthat/test-ggplot-hline-multiple-*.png`;

- ` tests/cookbook-test-suite/lines/basic_horizontal_line.*` has improved a little, but that's because the original ggplot2 code was broken--now fixed, and I'm not supporting x-ranges that are not numerical--to do;

- @chriddyp `hline_on_facets-*.png` now fixed (https://plot.ly/~TestBot/690).

/cc @pedrodz 
Oops, this comment should just go away...