Support partial recompiles

[mgsloan]

For iterating on compile errors within a single file, it can be advantageous to ignore all the downstream modules, instead . For example, this is useful for things like emacs flycheck.

I'm not sure how this should be implemented, but it might make sense to make it into an extension of the UpdateTargets interface. Discussing it with @chrisdone, who suggested adding this feature, he pointed out that it looks like it'd be sufficient to use LoadUpTo mn at this portion of the code

One thing to consider is whether the info for downstream modules should be invalidated - see this relevant issue / comment: #41 (comment)