BibFormat: new configuration of full-text snippets

• New configuration of full-text snippet facility with limits based on characters rather than words. Changes configuration syntax to allow for multiple snippet settings for multiple document types. The limits on the number of snippets to show and on the number of characters to show in each snippet can now be set differently for every different document status type. Useful in case of different agreements with different publishers. (closes #45)

Co-authored-by: Tibor Simko <tibor.simko@cern.ch>