Add new ggplot2 lesson

[aays]

• Renamed the old lesson material + lesson.md to ggplot2-apr-2019 and lesson-apr-2019
• New lesson just covers geom_point, geom_histogram, and geom_smooth before going into themes, legend formatting, plot annotation, faceting, patchwork, and ggsave

I feel like geom_smooth could be removed here in the interest of time - the lesson's a bit bloated as it is. That, or it could be cut down to just a single mention during the bit about adding an R^2 value to a plot. Thoughts?

[aays]

So I made a few other changes on top of the above:

• Set color aesthetic in geom_point when creating object so that a second object doesn't need to be made for the model fitting section
• Remove leftover mention of legend.key in theme prior to legend formatting section
• Demo axis.text.x as well to show example of theme inheritance (i.e. axis.text vs axis.text.x)
• Create a legend_theme object to make that persist through the lesson
• Add quick element_rect example
• Fix some typos in the plotmath section
• Add a brief coord_cartesian example at the end of the plot annotation section
• Mention other theme settings throughout the lesson where relevant (strip.background, plot.tag, etc)