Add a blueprint for generating a default config/ember-try.js

[ghost]

This would make the setup process for ember-try a bit easier.

(Sorry for this, I just saw the todo list at the bottom of the README.md)

[ming-codes]

Side effect of this is making upgrade ember-try easier.

[kategengler]

The config has never been backwards-incompatible.

There's now a command to get the config that is being used by ember-try (ember try:config), which can then be copied to config/ember-try.js and modified. And there's now way to configure without using scenarios (see versionCompatibility in Readme) such that the config is unnecessary for most addons. I feel this may be sufficient as the config should be irrelevant for many users.