Accept the RECORD environment variable to switch record mode

I'd like to add the ability to switch record mode from command line.
When I working on adding new test suite with TDD cycle, I have to switch `record` mode everytime.
That's pain. And I often commit `record: :all` option by mistake.

So, I'd like to switch record mode temporarily without changing code.
This can be done with `$ RECORD=all rspec spec/under_development_spec.rb:15`.