Make --join the default

When running lots of benchmarks, the first time I run them I invariably forget to include `--join` on my command line invocation, resulting in the benchmark results being spread across a long history of spewed output. I then take the time to run all the benchmarks again, this time with `--join`, as doing so is much less time consuming then finding all the results and manually cobbling them together into one table. Personally, I've never once wanted the non-joined behavior, and it'd be great from my perspective if `--join` were the default.