Added delimited_writer (for tab-delimited files) and csv writer 

Two new writers, with a little cleanup of `traject/line_writer.rb`

* DelimitedWriter will write delimited records, one per line, with defaults
  that produce tab-delimited rows with pipes for intra-field delineation.
* CSVWriter wraps it up in the stock CSV library

Both require at least one setting: `delimited_writer.field` is a
comma-delimited list of the field names to be output, in the order you
want them written.

Added both writers and some simple tests.