BibFormat: full-text snippet selection improved

• grep only for exact matches or in case of absense of those, match words beginning with the stem of a pattern
• avoid highlighting substrings which are parts of non-pattern words
• while cutting out the snippet, use re for matching patterns insread of the previous method
• ',' or '-' are properly recognized as word boundary
• "disappearing snippets" problem is fixed (it occured for multi-word search patterns) (see #272)
• in the input strings, the parenthesis are removed except in case there is no space between them, e.g. U(1)
• the highlight method is now also using the new highlight_matches method and "escapes" search terms