Implement more automated testing

I'm already using `check()` and `test()` but there could be more;

Add `lintr` checks. It's possible to add an automated test:

```
if (requireNamespace("lintr", quietly = TRUE)) {
  context("lints")
  test_that("Package Style", {
    lintr::expect_lint_free()
  })
}
```

Add `covr` checks. Still not 100% sure what this analyses though.
