support skip fields for bibtex export

see #78

README.md

@@ -497,7 +497,7 @@ License
 
 Jekyll-Scholar is distributed under the same license as Jekyll.
 
-Copyright (c) 2011-2014 [Sylvester Keil](http://sylvester.keil.or.at/)
+Copyright (c) 2011-2015 [Sylvester Keil](http://sylvester.keil.or.at/)
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the 'Software'), to deal

lib/jekyll/scholar/defaults.rb

@@ -15,6 +15,7 @@ module Jekyll
 
       'bibtex_options'        => { :strip => false, :parse_months => true },
       'bibtex_filters'        => [ :latex ],
+      'bibtex_skip_fields'    => [ :abstract, :month_numeric ],
 
       'replace_strings'       => true,
       'join_strings'          => true,

lib/jekyll/scholar/utilities.rb

@@ -275,12 +275,16 @@ module Jekyll
         e['key'] = entry.key
         e['type'] = entry.type.to_s
 
-        if entry.field?(:abstract)
+        if entry.field_names(config['bibtex_skip_fields']).empty?
+          e['bibtex'] = entry.to_s
+        else
           tmp = entry.dup
-          tmp.delete :abstract
+
+          config['bibtex_skip_fields'].each do |name|
+            tmp.delete name if tmp.field?(name)
+          end
+
           e['bibtex'] = tmp.to_s
-        else
-          e['bibtex'] = entry.to_s
         end
 
         entry.fields.each do |key, value|
@@ -291,6 +295,9 @@ module Jekyll
         e
       end
 
+      def bibtex_skip_fields
+      end
+
       def generate_details?
         site.layouts.key?(File.basename(config['details_layout'], '.html'))
       end