Multi themes

[amaury-hanser]

Questions	Answers
Description?	Update to Gulp 4, Mjml 4 and add multi-themes support
Type?	improvement
BC breaks?	yes
How to test?	Simply move your theme src in it's own subfolder src/foo/

I've base my work on the pull request #9 of @Scritik. I've updated the gulpfile.js to add multi-themes support.

You can run:

• gulp watch (will watch every subfolders of src/)
• gulp build (will build every subfolders of src/)

You can also watch and build per subfolder by running:

• gulp watch:foo
• gulp build:foo