Add options
- `stream-stdout-in-response`
- `stream-stdout-in-response-on-error`
- `stream-command-kill-grace-period-seconds`
to allow defining webhooks which dynamically stream large content back to the
requestor. This allows the creation of download endpoints from scripts, i.e.
running a `git archive` command or a database dump from a docker container,
without needing to buffer up the original.
There's a lot in this commit.
1. Add `pass-environment-to-command` option that works much like
`pass-arguments-to-command`. You can see an example usage in the
"github" test case.
2. Add a test program called "hookecho" that is used to test the
webhook package instead of relying upon a system `echo` command.
3. Move hooks_test.json to a template so that we can update the path to
hookecho on the fly.
4. Don't return an error at the end of hook.MatchRule.Evaluate(). All
tests succeed for me now.
