- Collected in animate_options.R
- Sets global defaults in plot_settings environment
- Adds ability to set animation options, similar to theme_set() from ggplot2
- Adds getters for current (if set) or default animation options

Merge branch 'pkg-add-anim-options-merge' into pkg-add-anim-options

# Conflicts:
#	NAMESPACE
#	R/animate_tidyr.R
#	R/plot_helpers.R
#	man/anim_options.Rd
#	man/animate_gather.Rd
#	man/animate_spread.Rd
#	man/static_plot.Rd

Changed name of set_text_color() to choose_text_color() to avoid confusion with the global option setter functions, e.g. set_text_size(). Updated tests to reflect modified threshold.

Uses the algorithm mentioned at https://stackoverflow.com/a/3943023/2022615 based on the luminance of the background color to determine black or white text. The threshold favored black text in a way that didn't look good with our plots, so I increased the threshold from 0.179 to 0.333.