Check model configs

[beckjake]

This is mostly @clrcrl's PR #725, with additional tests and rebased into dev/guion-bluford. No real code changes from her PR.

@clrcrl I can't request a review from you, unfortunately, but please give any feedback you have as well!

[drewbanin]

I dropped a comment in here about disabled models, but otherwise this looks pretty good to me!

Some other thoughts:

• we should get the tests passing
• I couldn't check out this branch? I'm not sure if it's because i have a local branch with the same prefix, or if it's something else, but I'm pretty confused. I ended up checking out the latest commit to get it working locally.

This should be ready to merge once we've discussed (and possibly acted upon) the enabled: false conversation.

[drewbanin]

There's an unfortunate effect that happens here:

Disabling models in dbt_project.yml file will effectively "remove" the models from the manifest, therefore causing dbt to think that the config is useless.

For example:

models:
    my_project:
            enabled: false

If your project name is my_project, this will (errantly) result in:

WARNING: Configuration paths exist in your dbt_project.yml file which do not apply to any resources.
There are 1 unused configuration paths:
- models.my_project

I don't really know that there's a good fix for this, as we've consciously decided to discard disabled resources in parsing. Would it be crazy to skip adding paths in get_unused_resource_config_paths if there's an enabled: false present? I think that would probably get pretty messy....

[beckjake]

Ok, I got this done. I had to refactor the loader code quite a bit, the ResourceLoader abstraction ended up getting in the way quite a bit, so now the GraphLoader just explicitly builds up a manifest as it goes. I think if we want to do more things like this (retaining extra data as we load/parse) we'll have to refactor the parser/loader code further.

[drewbanin]

I tried to break this as hard as I could, and I couldn't do it! LGTM!

Really glad to finally get this merged -- I think it'll be a huge help for new dbt users. Thanks for getting this over the line @beckjake and thanks so much for the original PR @clrcrl!!