Add support for per-line open/closing comments.

- Add a new configuration field 'afterEachLine' which contains a values that gets attached
  to each line in the license header (matches beforeEachLine).
- add new configuration field 'padLines' which is a boolean. If true, all the lines in the
  license header get padded to the same length.
- add new style 'XML_PER_LINE' which generates license headers that use per-line comments:

  <!-- this is the first license line -->
  <!-- this is another line           -->
  <!--                                -->
  <!-- above is a blank line          -->

- make it possible to omit each of the firstLine, endLine, beforeEachLine and afterEachLine;
  if they are missing assume that they are the empty string.

- add test for xml_per_line
- add docs.