add :preamble option to compiler

Per this thread:

https://groups.google.com/forum/#!searchin/clojurescript/preamble/clojurescript/rUIlLfcYZvE/Yskfh4znL_0J

1) reamble 's value will be a vector of paths
2) the compiled output is prepended with the contents of the files at those paths
3) the generated source map points to the correct/adjusted line numbers

Additionally, when compiling for the :nodejs target the preamble contents will default to the hashbang we currently write in that situation.