Improve Accessibility using Email Markup Consortium Recommendations

[garrettjohnson]

Is your feature request related to a problem? Please describe.
The email markup consortium has created an accessibility report and some of the top 10 issues are present in MJML, and could be fixed relatively easily.

Describe the solution you'd like

• Fix issue 1 - Content inside the body should be wrapped in a lang attribute.
  • Can be addressed by duplicating the langAttribute tag onto the body and updating the lang default to und as recommended.
• Fix issue 3 - Content inside the body should be wrapped in a dir attribute
  • Can also be addressed by cloning the dirAttribute tag and updating the dir default to auto as recommended
• Fix issue 7 - Images must have alternate text
  • Set the default alt text to a blank string, using defaultAttributes, so that mjml-image always renders correctly. This default could also be implemented on mjml-social-element, and mj-carousel-image.

I'm happy to submit a PR. Thanks for a great tool.

[iRyusa]

Sorry for the late reply but a big yes for a PR about those change 👍