[proposed] cli commands

There are 2 contexts where you can run a pie command:

• at the root of a pie package - the default context
• in a pie item directory - the item context

I propose that the commands allow defining these context.

The implicit context is pie. The proposed command structure is: pie [context] [cmd] [...options]

default context

• pie preview - run server w/ catalog ui, displaying readme etc, uses docs/demo as the item src
• pie publish - build catalog archive and send to catalog host
• pie clean - clean docs/demo

item context

To call commands in the item context you add item as the context: pie item ...

• pie item pack - pack the item
• pie item clean - clean the item
• pie item preview - run server that renders the item

Some alternate proposals:

https://gist.github.com/edeustace/08be55678f33a04c228fb302a17cc420

https://gist.github.com/evaneus/baea8efa89006263e3354122a283389c