Ignore figures

The .png files in the `figure/` directory are only used when the `.Rmd` file is weaved into a `.md` file. They are then encoded directly in the `.html` file, and as such aren't necessary to keep in the repository. Because they bloat up the repo size and very little useful can be done with them from a git perspective, I deleted the existing figures and added `figure/` to the `.gitignore` for the repository.

CC: @sgagnon