Project style detection

[justinfagnani]

This is an umbrella issue for project style detection: determining whether a project is a reusable element styl project, or application style project.

With this information we can differ the behavior and UX of a few sub-commands:

1. serve should should only the appropriate URL, and server -o should open the right URL
2. lint should warn on component directories in imports in element projects, and the lack of in application projects
3. build should warn on building elements, or we can figure out a sensible subset of build steps to run.

[justinfagnani]

Related: https://github.com/Polymer/polymer-cli/issues/778

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.