GFF/GTF parsing from cufflinks as a library

 This is a simple "libraryfication" of the GFF/GTF parsing code that is used in
 the Cufflinks codebase. There are not many (any?) relatively lightweight
 GTF/GFF parsers exposing a C++ interface, and the goal of this library is to
 provide this functionality without the necessity of drawing in a heavy-weight
 dependency like SeqAn.