This addresses some issues with joining fields, field values, and subfield text into strings. Rather than handle this using simple arguments such as field_delimiter and subfield_delimiter, there's a new class called TextStyle which handles all the merging. This makes it simpler for users to do what they want. If they want field to be formatted like this:
ind1=0 ind2=1 $a"Medicine" $b"for the people"
Well, they can do that by subclassing BaseTextStyle or TextStyle.