Add email-designer plugin

[alexzaganelli]

I'm happy to contribute to the Strapi community plugin's growth w/ this piece of software I've just created.

By submitting this pull request, I promise that:

• [x] I have read the contribution guidelines thoroughly.
• [x] I ensure my submission follows each and every point.

[alexzaganelli]

Is there something else I have to do? Thank you