Added html header to fetch Msonormal styles from Office document

[Philldomd]

Pull Request Description

This change will help Outlook client to recreate the message after the link is added in the bottom of the body. For more explanation and links see my answer to this stackoverflow: https://stackoverflow.com/questions/51603265/line-spacing-increased-issue-when-updating-email-body-with-office-js/78542353#78542353

Fixes #31

Checklist

• [x] Changes are limited to a single goal (avoid scope creep)
• [x] I confirm that I have read any Contribution and Development guidelines (CONTRIBUTING and DEVELOPMENT) and are following their suggestions.
• [x] I confirm that I wrote and/or have the right to submit the contents of my Pull Request, by agreeing to the Developer Certificate of Origin, (adding a 'sign-off' to my commits).

[Ayko1595]

Thanks for your contribution! I edited your comment to reference the issue that this PR fixes 😊

[Ayko1595]

This looks good!! However, after making our Contribution guidelines we decided that commits should contain a reference of the issue that it solves, and they should also be signed off and verified. Could you perhaps make these changes and commit amend your last commit?