officer 0.5.1

Features

• export shape_properties_tags() for pptx extensions.

Issues

• Manage also even/first/default sections not defined with officer (indicate
  there is even/first/default parts).
• Fix behavior regression with headers and footers replace_text_at_bkm. The previous behavior "don't trigger an error if a bookmark is missing" is back.
• force Word default continuous section if no type is specified and first page footer/header if it is seen.
• drop "Compatibility Mode" from Word documents.

Changes

• Change license to MIT.
• use images sha1 value for filename with package 'openssl'. This should fix issues with duplicated images basenames, or non utf names, etc.