Leads to better documentation where it is easier to understand how to use the anim_opts parameter

- 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 update documentation for both options

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.

- Use quasiquotation to get variable names
- Unified documentation
- Use match.arg to guard for correct arguments in type and export
- Removed animate_helpers.R

And change Long/Wide to lowercase to reflect that they are both variables names and a description of the state of the data.

animate_join() now lives in animate_join.R with the other animate_join_ functions. These all use rlang::enquo() to capture their arguments so that animate_join() can print the variable name in the plot title.

Sets figure output to man/figures/tidyexplain- and ensures that all R chunks that output figures are named. This gives a collection of animations and static images with reasonable names ready for download and use.

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().

- 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()

this makes it easier to differentiate between the join-related and the (coming) tidyr-related functions that have similar goals