Implement the debug flag and help command

I'm vacillating about the choice to have separate Config structs in the
`helm` and `run` packages. I can't tell whether it's "good separation of
concerns" or "cumbersome and over-engineered." It seems appropriate at
the moment, though.

README.md

@@ -6,7 +6,7 @@ TODO:
 * [x] Make a `Dockerfile` that's sufficient for launching the built image
 * [x] Make `cmd/drone-helm/main.go` actually invoke `helm`
 * [x] Make `golint` part of the build process (and make it pass)
-* [ ] Implement debug output
+* [x] Implement debug output
 * [ ] Flesh out `helm upgrade` until it's capable of working
 * [ ] Implement config settings for `upgrade`
 * [ ] Implement `helm lint`