{} could be matched arbitrarily inside of a {} pattern, in addition to
the expected behaviour of nested pairs. (oops)

Closes #143

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