Not compatible with MU

[simonihmig]

Two issues:

• the templates tree given in lintTree seems to be empty in a MU project. I wrapped tree (here: https://github.com/ember-template-lint/ember-cli-template-lint/blob/master/index.js#L55) in a debug(), which wrote an EMPTY_MERGE_TREE
• the ember template-lint:print-failing fails because of a hardcoded app/ path:
  https://github.com/ember-template-lint/ember-cli-template-lint/blob/master/lib/commands/print-failing.js#L21