Implement command `ks registry add`

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>