Wraps pandocs bibiliography handling

In order to add a bibliography, you will need a bibliography file (e.g. .bib) and a CSL file (.csl). Both need to be compiled with their respective compilers (biblioCompiler and cslCompiler). Then, you can refer to these files when you use readPandocBiblio. This function also takes the reader options for completeness -- you can use defaultHakyllReaderOptions if you're unsure. pandocBiblioCompiler is a convenience wrapper which works like pandocCompiler, but also takes paths to compiled bibliography and csl files.