Add a way to pass extra arguments to RSpec

This way we can easily set up on Jenkins two separate builds, that will
(or will not) run the specs with a particular tag:

blaze EXTRA_RSPEC_ARGS="--tag '~external_connections'"
blaze EXTRA_RSPEC_ARGS="--tag 'external_connections'"
Merge pull request #8 from murbanski/master

Extra arguments to RSpec