Copy MJML to MJML Playground

**Is your feature request related to a problem? Please describe.**
No.

**Describe the solution you'd like**
Since we might use '<mj-include>' in app, If we can "Copy full MJML" 
then we can paste in [MJML](https://mjml.io/try-it-live/) for others to quick test.

**Describe alternatives you've considered**
No.

**Additional context**
No.