Add a way to mark slow tests and exclude them by default

[sol]

e.g. by tagging tests or something

[dnadales]

Adding my vote, as this will be really helpful for us at TorXakis. We have a compiler tests that takes a large file, and we'd like to avoid running this while running stack --file-watch :)

CC: @pjljvandelaar.

[sol]

I think the following could make a nice API for this feature...

tag :: String -> SpecWith a -> SpecWith a
tag = undefined

...which then can be used like so...

it "behaves according to the spec" >>> tag "slow" >>> tag "db" $ do
  ...