Show all countries in data set

This patch changes the list of countries to be dynamic, based on the unique countries listed in the data set, so new countries (hopefully no more) will show up when they're added to the list.

There are also a couple of stylistic changes; I've slightly changed the loading code but it doesn't change its behaviour. I've also set the `ggplot` theme to `minimal` as the yellow in Color Brewer doesn't show up too well on the default grey background.

The app's deployed [here](https://mathew-hall.shinyapps.io/shinyCountryTimeseries/) if you want to see it running.