Report client-go version in `version` subcommand

Make kubecfg diff exit with non-zero code when a diff is found.

Add manifestJson and manifestYaml functions

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.

Set appropriate delete option for server version

Annoyingly, delete options `OrphanDependents` and `PropagationPolicy`
are mutually exclusive.  This change compares the k8s server version
to see if the server is going to look for the newer PropagationPolicy.

Fixes a78be141 (aka PR #59)

Update to client-go v3.0.0

Set delete propagation policy (to "foreground")

Kubernetes 1.6 adds a `PpropagationPolicy` to the delete options,
obsoleting the previous `OrphanDependents` boolean.

This change sets `PropagationPolicy=foreground`, keeping the legacy
`OrphanDependents=false` for older k8s servers.

Note that PropagationPolicy=background is broken in k8s 1.6 and (eg)
leaks pods from a deployment.  See kubernetes/kubernetes#44046 for
upstream discussion.

Add 'subset' diff stategy option that checks the live state is a subset of the config state.

Anonymously import the GCP auth provider to get the registration side effect.

Register *all* auth plugins (gcp + oidc)

Fix deploy runtime version check harder

Travis' deploy.on golang version check seems to be surprising.  In
particular `deploy.on.go=1.8.x` will *not* match a build performed
with `go=['1.8.x']`.

This change just gives up and uses an explicit bash conditional.
Uglier, but fewer surprises.

Update to newer xcode for travis osx builds

Frequent `make test` crashes on (default) xcode7.3 in jsonnet_cgo. Use
a different/newer build environment.

Fix go point-release version references

Prefer `1.8.x` rather than `1.8`.  In particular `deploy.on.go=1.8` does
not match (eg) `1.8.3` :(

Update to client-go release-3

Switch logging from glog to logrus

Add per-file license headers

Improve interactive output experience by switching to a logging
library (logrus) that allows customising the output format.

This also removes the glog command line flags.  Replaced with a
simpler `--verbose`/`-v` option:
- quiet by default (warnings and errors only)
- `-v` for progress info
- `-vv` for debug

Fixes #34

Fixes #35

More complete implementation of extvars, topvars