Fixes #108.

For example, 'apply <env' currently operates as a no-op. With the
introduction of simple environments in PR #131, 'apply <env>' should
perform basic validation such that:

1. The user has added the environment that is being deployed against to
their Ksonnet project.

2. The URI in the environment's spec file that the user wishes to deploy
to should correspond to at least one cluster location as listed in
kubeconfig.

If either of those conditions are not satisfied, the kubecfg user will
receive the corresponding error.

In addition, this commit will set the kubectl --cluster flag to point at
the cluster listed by the environment URI.

Currently, if the user wants to deploy a ksonnet application, and that
application uses some vendored library, or something in the `lib/`
directory, they will have to pass the appropriate `-J` flags into the
command themselves.

This commit will automatically add these whenever we're in an app
directory and a command is issued. In particular, even if the user
passes in the `-f` flag (rather than an environment name), we'll still
add library paths to the command if we're in a ksonnet app directory.
This is meant to capture the case that a user wants to update one
resource in particular in a ksonnet application.

This reverts commit 020d5da0.

The change broke --namespace (and other flag) handling, because it
recreates the `clientConfig` global singleton multiple times - only
some of which are correctly tied to the relevant flags.

This commit will separate the application level logic for the 'show', 'delete',
'validate', and 'diff' commands from the Cobra logic in the cmd/ package.  The
application level logic will be placed in pkg/kubecfg/.

The ksonnet.next design doc specifies the core kubecfg verbs (i.e., the
ones listed above) to all have the form:

  kubecfg <verb> [<env-name>|-f <file-or-dir>]

That is to say, each of these should be able to take either an
environment name, or a `-f` flag with a list of files and directories to
apply `verb` on. In the case of the environment, we will apply `verb` to
every component in the `components/` directory.

This commit implements this behavior for all these verbs.

This commit will introduce the `template.Expander` abstraction, which is
meant to abstract over an invocation of the Jsonnet VM. Specifically, it
provides facilities for users to provide (e.g.) Jpaths, ext vars, and so
on.

The main justification for this change is:

* We need a common way for the `pkg` and `cmd` packages to interact with
  the Jsonnet VM.
* The `utils` package is already too much of a catch-all.
* It's easier to think about an invocation of the Jsonnet VM when we
  additionally encapsulate the parameters we pass to it on every
  invocation in kubecfg.

Fixes #35

- Support the standard client-go/kubectl flags
- Switch object types to use client-go rather than apimachinery
- Implement update subcommand

File format is recognised via simple file extension check.

Any "List" objects found will be expanded into their constituent
items, so single-element formats like json and jsonnet can use a
v1.List to capture multiple resource objects.

Basic skeleton.  Can evaluate jsonnet and show the output as yaml or
json .. and that's about it.