more in line with set_text_* functions

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

And demonstrates usage with the tidyr gather/spread animators. In essence, user-facing functions take an argument `anim_opts = anim_options()` as a last argument (after ... if applicable). These options should be passed through to lower level functions and should be given to `animate_plot()` and `static_plot()`.

This implementation actually doesn't break previous functions, but the join and set families will need to be updated to match.

Currently only used for setting global text and title font sizes, but can be extended to other options that we may wish to set for more than one plot. For fonts, we now provide `set_font_size()` that can be used to set text and title sizes across all subsequent animations. Includes internal getter functions get_{text|title}_size().

Adds a few additional dplyr functions to the package imports, in particular: slice, data_frame, and row_number.

- Import packages from tidyverse
- Set global imports in zzzz-package.R
- Import pipe and tidyeval
- Update authors
- Downgrade version to 0.0.1.9000
- Change license to MIT
- TODO: Internal package qualifications still needed, e.g. dplyr::row_numbers()