refextract: Improvements to IBID identification and editor notation.

• Upon finding a list of editors in a reference line, if there exists more than one editor, the notation used when marking them up with be 'eds.' as opposed to just 'ed.'.

• The expression handling the recognition of IBID's from pure journal numeration now takes into account the presence of series letter. (Not including the series letter was blocking many automatic recognition of IBID's).

• The proper series letter is now used, for IBID's explicitly marked as an IBID in the reference. This was an overlapping problem concerning the checking between the word 'IBID' and 'IBIDEM', but now rectified using negative lookahead assertion.

• Also added another nice ibidem example to the list of test references.

• Some leading whitespace has been removed.