Use two-digit identifier for generated figures

[gvwilson]

@fmichonneau writes:

For the fig path, I think it has been inconsistent. Personally, I
think it's a good idea to given figures a prefix for each episode, so
we can see right away where the figures are coming from (and whether
some need to be deleted if they are not in use anymore). Therefore, in
the gapminder lesson, I set as a prefix the 2 digit number for the
episode (even if the lesson doesn't generate a figure), so each
episode should start with something like:

    ```{r, include=FALSE}
    source("../bin/chunk-options.R")
    knitr_fig_path("01-")
    ```

1. Make this change to all .Rmd files.
2. Clean out all old figures.

See also swcarpentry/r-novice-gapminder#166

[butterflyskip]

Hi,
A couple of questions:

1. Should I include _episodes_rmd/.Rhistory to the PR? (included it for now)
2. What are considered "old figures" referred to above? I have not done that part in this PR