Currently users are unable to add their own registries through the CLI.
This limits them to a small subset of prototypes found in the default
incubator registry.

This commit will add the command `ks registry add`, that allows users to
add registries supporting the `github` protocol.

It will be of the form `ks registry add <registry-name> <registry-uri>
[--version]`. If a version is not specified, `latest` will be used.

Signed-off-by: Jessica Yuen <im.jessicayuen@gmail.com>

Signed-off-by: Jessica Yao <jessica@heptio.com>

Signed-off-by: Jessica Yao <jessica@heptio.com>

Currently, commands such as `ks env non-existent-subcommand` will output
an error `Command 'env' requires a subcommand`.

This is not helpful since the user may assume there is the subcommand
'non-existent-subcommand'. This commit will clarify the error messages
in this scenario.

Signed-off-by: Jessica Yuen <im.jessicayuen@gmail.com>

This commit will partially resolve #38.

`registry list` and `registry describe` were designed to behave very
similarly to `prototype list` and `prototype describe`. The former
command is self-explanatory, and the second does displays some
documentation about the object in question.

This commit will implement these commands per the spec, and is a partial
fix to #38.