Closes #67
Thanks @andrewpbray

- Move long intro to pkgdown
- README is now much shorter
- Rename pkgdown "Get Started" to "Features"
- Add ggplot theme to quick intro
- Add pkgdown docs link to description

- Moved images to assets branch
- Moved rmd fragments to man/fragments

* e.g. style_duo() <- duo()
* style_xaringan <- write_xaringan_theme()
* style_extra_css <- write_extra_css()
* added NEWS entry
* updated docs and /docs