ggplot2 theme, color additions

[eroten]

• New function theme_council() follows the ggplot2 theme_... convention. Function also has complete ggplot2::theme() arguments, which makes further customization easier.
• New vignettes for using options and color examples
• council_theme() deprecated, with accompanying warning.
• Add blue_cascade list with shades of blue and corresponding colors for background and text cascading menus.
• Update documentation and acknowledgements
• Remove footnote from RMarkdown template

closes #23 closes #22

[ehesch]

👀 yes! I'll get to this soon!

[ehesch]

@eroten please review the additional changes I pushed for theme_council() and adding a new theme_council_open(). Changes made based on what I have been asked to change in figs over the last several months. PS, do you know if we can force a y label to user stringr::str_wrap(width = 15) ? I think we cannot, but it would be ideal to adjust in the theme!