Add benchmark `baseline` option

[ntninja]

Add baseline option that determines (according to the included docs):

Whether this benchmark's results should be considered as possible
base line values when comparing them to other results of the same
group? Use this if you want to include some results just for
comparison, without them affecting the relative scores displayed
for other results.

[ntninja]

Anyway, would be cool if somebody could tell me why the test fails, cause looking at the details gives me a 404.

[ntninja]

Glad to hear back from you! Did you take a look at the CI yet?

[ntninja]

@ionelmc: Ping