cmd/bundle: add --no-secrets switch.

This is handy if you're creating a `brew bundle` environment that makes
use of Homebrew's environment filtering of secrets/tokents/etc:

`/(cookie|key|token|password|passphrase)/i`

While we're here, let's also add `HOMEBREW_BUNDLE_CHECK` and
`HOMEBREW_BUNDLE_NO_SECRETS` environment variables.