Change source dir to a sassify specific key

[Lasering]

Problem

Currently the sassify source dir is defined with:

Assets / sassify := Def.task {
  val sourceDir = (Assets / sourceDirectory).value
  val targetDir = (Assets / sassify / resourceManaged).value
  (...)
}

If I need to change the source dir for sassify I'm forced to change the sourceDirectory for all the sbt-web assets. This could be easily changed with:

val sourceDir = (Assets / sassify / sourceDirectory).value

Since Assets / sassify / sourceDirectory is not explicitly set sbt will search the "parent" Assets / sourceDirectory, thus producing the same behavior as the current code. However if the user sets Assets / sassify / sourceDirectory then the source directory will change just for sassify.

Why is this useful?

I want to have my .scss alongside with my .scala sources. This is especially helpful on frontend projects.