Reintroduce preambles in DSL

As a8f69a80 removed the preambles feature, we need to introduce a replacement within the new Kotlin DSL. This will be done in the course of #11 (closed).

Suggestion for the replacement:

val mypreamble = listOf("pdflatex", "bibtex", "pdflatex", "pdflatex")
configure(file("quack.tex")) {
  directives = mypreamble
}
configure(file("blub.tex")) {
  directives = mypreamble
}