Check that the danger invocation isn't straight danger

[hawkrives]

Addresses #470

Checks that the danger command was invoked by way of some subcommand.

[orta]

I did not know about the sum function, damn, I could have wrote a lot of code better with that 👍

[orta]

Thanks @hawkrives

[peril-staging]

Thanks for the PR @hawkrives.

We conform to the Moya Community Continuity Guidelines, which means
that we want to offer any contributor the ability to control their destiny.

So, we've sent you an org invite - thanks 🎉

Generated by 🚫 dangerJS

[orta]

Tested this locally, because commander mutates process.env this didn't work - fixed it in #469

[hawkrives]

Oh right, you mentioned that. Thanks.

[peril-staging]

Thanks for the PR @hawkrives.

This PR has been shipped in v3.0.1 - CHANGELOG.