Allow selecting benchmarks to run using tags

[travisdowns]

Currently the benchmarks to run are organized into a hierarchy and you can select a subset to run using --test-name=PATTERN, but we should have a cross cutting tag feature that allows you to select by tag.

For example, there could be a default tag that picks out the most interesting tests to run by default. This in particular will solve the issue where you get a ton of very specific tests (e.g., for the memory disambiguation stuff) spammed to the output.