Add custom (or extensible) TestThat reporter

[colearendt]

The ListReporter does not allow caching additional information when tests execute... it would be nice to be able to do so, in order that the definition of the code we are executing could be captured, as well as the output that is being compared.

Motivation:

Remove "code" from the test name for a cleaner ggplot presentation. The "code" is still available in the testthat_results object

well... not completely. Not sure how to capture the output of what manip is defined as...

[colearendt]

We should be able to stash:

• query generated
• code executed (new stack trace stuff?)
• output from local
• output from remote
• any error message, etc.

If all of this is presented in a nice fashion, I think we will be in a really good state for iterating quickly on these tests!