add citeproc to the bundle

citeproc is used to handle generation of bibliography entries from bibtex files.
convert one document to asciidoctor-compatible citation statement

this file will be used as a test case.
copy bibtex file here, and use underscore filename

the bib file is only used to generate bibliography, will not be
uploaded to the generated dir.
add gems for bibtex
half-baked solution

I want to use extension to implement the citation processing
feature. But the legacy setup from Hakyll, i.e. per page settings
for csl style and bib file doesn't play well with Awestruct's
model. I will need to set site-wide bib and csl style to use the
extension. That will make more sense.
add sitewide bibliography support with citeproc