[generalize] Make SELFTIMED style tags editable in a config file.

[rrnewton]

This protocol of tagged lines is growing kind of large, and it should be abstracted.

Also, as a preemptive measure we should probably add even more fields to the benchmark result schema so as to have room for people to shoehorn in weird stuff. Either that or make the schema itself extensible.

[rrnewton]

Also, when we get around to this, some of the hackier extensions to the core schema should be done away with -- productivity, jittime, etc. Those are two environment specific. All that should stay in the core are universal things like TIME, and data about the host machine.

All backends are going to need to be extended with the ability to add custom columns then...

[rrnewton]

Update: custom tags are now supported, but they are done right in the Config value. We have not gone back to having a text-based config file (yet).

[rrnewton]

New use case for this -- in Pycket-benchmarks we want to be able to override SELFTIMED="RESULT-cpu" (cc @samth)