Add ggtaichi extension

[PursuitOfDataScience]

This PR adds ggtaichi to the gallery.

ggtaichi is a ggplot2 extension that compares two data sources on a single grid of taichi (yin-yang) diagrams: geom_taichi() turns every cell of a heatmap-like grid into a taichi symbol whose two interlocking fish are filled by two different sources, so four dimensions can be shown on one plot. It is the sibling of ggDoubleHeat (already in the gallery) and reuses its two-scale design.

• Repo: https://github.com/PursuitOfDataScience/ggtaichi
• Added a 350x300 PNG thumbnail at images/PursuitOfDataScience-ggtaichi.png
• Added one entry under widgets: in _config.yml

Note: a CRAN submission is currently under review, so I set cran: false for now; happy to flip it to true once it is accepted. The examples link points to the pkgdown article, which goes live once GitHub Pages finishes its first deploy.

[yutannihilation]

Thanks!