This permits custom attributes to be passed to the bibliography list
such as `reversed` or `start` to permit custom numbering.

See inukshuk#148

This filter converts \textsuperscript latex formatting into <sup> html
tags. It should be used before the `latex` filter to prevent premature
stripping of {} groups.

Also adds superscript to the default filters and documents the
bibtex_filters parameter in README.md

References inukshuk/jekyll-scholar#132

 * Add group_by option, to group bibliography entries.
 * Add proper names for entry types, and months.
 * Add support for multilevel sorting.

Removed tag option for raw bibtex entry.
Added a note in the documentation on the usage of this option.

see #78

Instead of the 'numbering' option, 'bibliography_list_tag' and 'bibliography_item_tag' have been introduced. Defaults.rb has been updated.

add support for file repositories

fixes #37

fixes #17

see #16

#19