Only test dimensions that are changed

[joshtemple]

Now that we have the ability to parse LookML in Python with lkml, we have the ability to compare the differences in the parsed tree and only run Spectacles against those modified dimensions.

I'm imagining running git diff --name-only $TARGET_BRANCH to determine which files were changed (it doesn't look like Circle has a target branch environment variable in its context but it is a highly requested issue).

For each changed file, we would parse the LookML on the source and target branches and compare the resulting trees to understand which dimensions were changed.

Last, we would need a way to pass specific dimensions to Spectacles via the model selector (might require changes to requirements in #21). Fonz then would query only the modified dimensions.

This could also interact with caching in #22. You might want to run the unchanged dimensions using the cached queries instead of rebuilding them.

[DylanBaker]

This all sounds good to me.

Last, we would need a way to pass specific dimensions to Spectacles via the model selector (might require changes to requirements in #21). Spectacles then would query only the modified dimensions.

I agree it might require some changes there. I can post comments on #21, but my gut feel is simply to extend the model selection with another '.'? i.e. model would be model.*, explore would be model.explore.* and dimension would be model.explore.dimension? It's possible that gets too verbose?

One thing I think we would probably need to do is also run any dimensions that reference the changed dimensions? I think we could also pull that from the parser?