ggplot: geom_text()

Basic implementation of geom_text()

Arguments supported in this PR:
Color: https://plot.ly/~pdespouy/1248/mpg-vs-wt/; 
Size: https://plot.ly/~pdespouy/1249/mpg-vs-wt/; and, 
Combination of both: https://plot.ly/~pdespouy/1251/mpg-vs-wt/.

For angle, plotly support textangle for annotations, but not in traces. To be implemented. 
For position, translation is not direct: integers in ggplot vs text in plotly. To be implemented. 

//cc @mkcor @chriddyp 
Maybe you can take this out of the `test_that()` function, since you are using it again later.