allow external app types to run

[edeustace]

actions run against some internally defined apps: default, info, item, catalog. These apps do the install, prep the webpack config then do the build. They can prepare the bundle for a given context. for example catalog is used to prepare the bundle for use in the context of the pie-catalog web app. It is foreseeable that 3rd parties may want to prepare the bundle for another context, say if they want to add an integration point, or if they have a different build set up (they may have everything pre-installed and want to use that).

A first run at the cli api:

pie apps list
pie apps add my-app
pie apps rm my-app
pie pack -a my-app