more in line with set_text_* functions

before the additional arguments such as color_header  = "black" would trigger a warning that the argument is not recognised. Now the color arguments passed to add_color_join

theme_env seems to have been retired from ggplot, replaced with theme_void

And no need to filter out NULL elements

- 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