Add bibtex-order setting to allow for standard biblatex sorting options

[jimisola]

biblatex knows various schemes for sorting. From section 3.1.2:

The sorting order of the bibliography. Unless stated otherwise, the entries are sorted
in ascending order. The following choices are available by default:

nty Sort by name, title, year.
nyt Sort by name, year, title.
nyvt Sort by name, year, volume, title.
anyt Sort by alphabetic label, name, year, title.
anyvt Sort by alphabetic label, name, year, volume, title.
ynt Sort by year, name, title.
ydnt Sort by year (descending), name, title.
none Do not sort at all. All entries are processed in citation order.
count Sort in descending order of number of times cited
debug Sort by entry key. This is intended for debugging only.

I really like that AsciiDoc is standardized, i.e. there are no flavors. I'm therefore thinking that it would be nice if asciidoc-bibtex would be as similar to bibtex/biblatex as possible.