Allow plugins to use manifest.yml to infer the current app

* move manifest argument logic into default_to_app_from_manifest method 
* added 'fail_without_app' flag allow plugins to handle no-app case themselves

Signed-off-by: Glenn Oppegard <goppegard@pivotallabs.com>
Remove outdated README

Signed-off-by: Glenn Oppegard <goppegard@pivotallabs.com>
Make console command able to use manifest to determine app

* had to load plugin/manifest first so that dependent plugins can use it

Signed-off-by: Glenn Oppegard <goppegard@pivotallabs.com>