Implement 'prototype preview'

Currently the command 'prototype use' expands a prototype and prints to
stdout. This is useful, but most of the time, users want to simply dump
the result in 'components/'

This command implements this print-to-stdout behavior in a new command,
'prototype-preview', and reimplements 'prototype use' to drop the
expanded prototype into 'components/'.

The new form of this command is:

  ksonnet prototype use <prototype-name> <component-name> [type] [flags]

So, for example, a command like:

  ksonnet prototype use deployment nginx-depl [...]

would expand the 'deployment' prototype, and place it in
'components/nginx-depl.jsonnet' (since Jsonnet is the default template
expansion). Alternatively, something like this:

  ksonnet prototype use deployment nginx-depl yaml [...]

would expand the prototype and place it in 'components/nginx-depl.yaml'
(assuming that there is a YAML version of this template.