jsonnet's `std.toString()` returns "compact" JSON.  Sometimes this is
not appropriate, particularly when generating large output that might
need to be viewed/debugged by a human.

This change implements manifestYaml and manifestJson functions, which
generate YAML and "pretty" JSON respectively.

A slight implementation wrinkle is that libjsonnet currently only
supports passing primitive (scalar) types to native functions, so
these arguments need to be JSON-serialised/unserialised across the
native function call boundary.

Fixes #35

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

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

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

Fixes #12