Introduce 'component' commands, starting with 'component list'

This adds a high level 'component' command and a 'component list'
command. 'component list' will pretty print all the components in
ksonnet application directory.

To accomplish this, an API is added to the metadata manager that returns
all components. Components are the individual files in /components, with
the path extension trimmed.

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