Two issues are addressed in this commit:
1. Instead of only sending the predefined environment arguments, this
commit appends the arguments to the existing OS environment. Fixes#53.
2. If an argument is not found in the payload, allow the command to run
and pass in an empty string as a placeholder. Fixes#54.
Additionally, I replaced `hook.ErrInvalidPayloadSignature` with a new
`SignatureError` type so that we can embed the signature in the error.
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.
