#209 specify fields exporting to Bibtex

[xmliu]

The current function of "customize entry types" only
used for checking the integrity of Bibtex entry.
However, I hope it can directly affect the output of
Bibtex because I want to export only required fields
(and maybe some optional fields) to Bibtex to keep it
concise and readable.
Another request is to allow the user to specify some
words which should always be in uppercase in titles,
e.g. DNA and HIV. The problem is when I fetch some
entry from Medline, all captial letters are in {} so
that they'll be transformed to lowercase in some
style. Another solution is if there are two or more
continuous capital letters in a word, most likely they
are abbreviates, so do not put them into {}.
Thanks.