Add tests

[eliocamp]

Currently there are no test in the package and many examples are not run. covr::package_coverage() reports 0.15% coverage. I think that adding tests to increase coverage to at least 60% is necessary before publication (ideally it should be higher, but testing ggplot2 extensions can be hard since it involves a lot of visual testing; even ggplot2 barely reaches 80%).

Check the vdiffr package for a guide on how to use visual regression testing.

[jamesotto852]

We have implemented many unit tests in #31, covr::package_coverage() now reports 95.05% coverage.