Update render_possible_doi to match more valid DOIs

Possible fix for #139, although I'm unsure about the nested non-capturing groups in the RE, and if the '$value = $1' is the best way of doing this (it does seem to work though).

Based on ~7,000 id_numbers, of which ~6,000 were DOIs, which included variations like:
DOI: 10.xxxx
doi 10.xxxx
10.xxxx