add Workbook#num_vml_files
add Workbook#num_comment_files
Comments#sorted_comments : set @sorted_comments only when @sorted_comments.nil?
some refactoring.
workbook.rb : remove temporary variable 'tag'
Workbook#write_workbook : use tag_elements block.
add Utility#r_id_attributes
add Worksheet#row_attributes
remove Worksheet#write_single_row. because it doesn't used.
test for Worksheet#set_selection
bug fix in Worksheet#set_selection
regression/test_utf8_*.rb added.

 there's not test_utf8_02.rb because it contais write_utf16be.
test for issue where shapes on one worksheet corrupted charts on a subsequent worksheet and for issue where add_button invalidated cell comments in the same workbook.
fix for 2 issues (excel-writer-xlsx #52 and #64)

   ! Fix for issue where shapes on one worksheet corrupted charts on a
    subsequent worksheet. #52.

   ! Fix for issue where add_button() invalidated cell comments in the
     same workbook. #64.
Fix for leading = in condtional_formatting range.
add regression/test_chart_axis25.rb and pass.
Added chart axis font rotation.
added regression/test_table15.rb and pass.
Merge branch 'slim_worksheet'
set version to v0.69.0.