Add support for unmanagedSourceDirectories over unmanagedSources

[DomBlack]

At the moment the plugin is based on unmanagedSources which defaults to *.scala and *.java only. However the plugin allows us to specify headers by extension.

I believe the better solution would be to modify the plugin to use the unmanagedSourceDirectories key and find all the matching files in those directories based on the headerSettings map passed in, rather than getting the user to both specify the extension in the map, and then update their includeFilter in (Compile, unmanagedSources ) to include custom extensions too.

This is useful when dealing with plugins which use different file formats, but still can take copyright headers (such as .proto files and .txt files for i18n)

[saeta]

@hseeberger If I were to make a PR that implemented this, would it be merged in & a new release published? Or should we fork this project? Thanks!