Vignette for using Reporters

[jimhester]

The documentation for reporters is really sparse, particularly for how to use them in test(), test_package(). It is probably worth having a short vignette to explain how to use them / pass options to reporters, and maybe defining a new reporter.

[jimhester]

This was originally brought up by https://community.rstudio.com/t/good-resources-for-testthats-reporters/2102

[coatless]

@jimhester any eta on when this will likely happen?

[hadley]

The documentation was generally so poor that I think that's the place to start, rather than a vignette. It's still far from perfect, but it should be substantially better now.