Add ignored directories method to BasicBuild

[darthorimar]

Now BasicBuild contains only target directory which can be marked as ignored by IDE. May be useful to additionally add ingoredDirectories: Seq[File] method which by default contains Seq(target) It allow users to override this method to tell IDE what directory should not be considered as sources.
E.g. on cbt source code it would contains cache dir.

[cvogt]

Since the root directory of a project is not considered a source directory, only the .scala files inside of it to be sourceFiles, target was never included, so excluding it doesn't make sense for cbt's way of thinking about source files. I recognize that IntelliJ's model seems to only consider source directories, not files, so in order to get the .scala files in the root directory, you'd have to manage exclude of everything else. Let me think about this a bit more in order to find a solution for everything.