made searchplos and friends output an S3 class with attributes for a single output or a list of outputs, and output now a list, plos_todf added, closes #54, closes #51
removed returndf param from searchplos
added high_browse to browse highlighted sections, and highplos to search for highlighted text only
added fxn to browse highlights in browser, fixed bug in plosauthor
edits to egs in highplos and high_brow
Merge pull request #57 from ropensci/s3

searchplos now with output as class "plos"