Add ability to disable an entire module

[Daenyth]

My use case is that I have a multi-module repository, and I want to run this task on some, but not all, of the modules. I'd like it to be "opt out" so that new modules by default have it added, but that it gets skipped for the ones that shouldn't need it (this is related to #40)

Something like unusedCompileDependencies / enabled := false inside a module's .settings() block.

I think I should be able to put together a pull request that adds this behavior, if it's something you'd merge in

Right now it's possible to hack this in like so:

(project in file("blahblah")).settings(
  unusedCompileDependencies := Def.task(Set.empty[Dependency]).value,
)

[keynmol]

What about

checkUnusedDependencies := true/false
checkUndeclaredDependencies := true/false

?