Allow using more capabilities #391

I'm using ghcid to run an hspec testsuite. hspec allows to parallelize the test execution. However, by default, the testsuites only use one capability (as witnessed by getNumCapabilities). Which means, that the tests will only be executed in pseudo-parallel. I can change the behavior by calling e.g. getNumProcessors >>= setNumCapabilities before the tests. And with that I can observe a speed-up in test execution in an example testsuite. But that's a bit sad, for two reasons:

1. It means that +RTS -N$some_number options passed into the testsuite when compiled (i.e. not running through ghcid) will be ignored.
2. I have to remember to do this, whenever I want truly parallel test execution with ghcid.

Can I somehow pass in -N as an RTS option to ... well, I don't even know. ghci? Or --test ':main +RTS ...'? (The latter doesn't work.)

Also, should using more capabilities be the default?

My full ghcid invocation looks like this, if that matters:
ghcid --command "cabal repl test:spec" --test ':main' --warnings

See also: https://hspec.github.io/parallel-spec-execution.html

Thanks!