Export common functionality for interpreting a list of arguments

Hmm.. maybe it would be nice to move all non-specific functions to a erlydtl_utils module that both erlydtl_compiler and any extensions can use.

This will apart from cleaning up the compiler and not litter it with exporting "internal" functions make it more comprehensible, I think.. ;)

Bah, the above has nothing to do with this particular PR; more than noting in which direction we're headed, and perhaps time to do something about it..