Fix goreportcard-reported lint issues

- `gofmt -s utils/registry_test.go`
- Fix missing `if err != nil` check in `cmd/delete.go` (a bug, but not critical)
- Add goreportcard button to README

Replace quickstart example with something that works

Add several regex native functions

Rename --extVar to --ext-str

- `escapeStringRegex`
- `regexMatch`
- `regexSubst`

They do what you expect, as implemented by the golang `regexp` package.

For consistency with other options (hyphenated, not camel case), and
the upstream `jsonnet` tool.

This allows you to actually reach the guestbook in most environments
without further changes (including minikube, since LoadBalancer
implies NodePort).

Remove poor attempt at setup step entirely.  This makes the actual
quickstart content more obvious ... and anyone who _actually_ wants to
run it will already know how to checkout a git repo.

Update jsonnet-cgo to 04f8990

Update example to use something self-contained.

Add diff,delete to the quickstart example

fix README example

Add new `resolveImage` native function

Add a new native function that resolves docker image names into more
specific forms.  In particular, it can look up a docker registry and
convert image:tag to image@digest at jsonnet-eval time.

Limitations: Does not currently support private docker
registries (that require authentication).

Controlled via two new command line flags:
- `--resolve-images` Change implementation of resolveImage native
  function. One of: noop, registry (default "noop")
- `--resolve-images-error` Action when resolveImage fails. One of
  ignore,warn,error (default "warn")

Note in particular that the defaults will *not* do remote registry
lookups, and will only add an explicit ":latest" tag where no tag was
given.

Fixes #13

Add native convenience functions to parse JSON and YAML

Native function tests crash on golang 1.7/osx.  Unclear if this is a
golang/cgo bug on osx, or jsonnet_cgo is doing something incorrectly.
Not critical, provided the golang 1.8 builds continue to work.

Fixes #12

Adds support for native functions

Add a flag for extvars

Minor README updates

- Add travis build status badge
- Update description of current status
- Add note about upstream golang OSX+cgo issue with <1.8.1

Build release binaries for osx

Add direct links to jsonnet tutorial and stdlib

Implement diff subcommand

Uses github.com/yudai/gojsondiff for the heavy lifting.

Implement delete subcommand

This removes all objects listed in config files (and dependents) -
effectively reversing an `update --create`.

Intended for short-lived "try this out; now clean up" workflows.

Fixes #14

Make the install commands more cut+pasteable