Load templates via a directory or files

[sawa-ko]

Currently I see that the templates are loaded through a string, which I think is not ideal in the case of having many templates as they will be many lines of String code, and I think having a function to register the templates based on a directory or specifying a specific file would be better. Since you could also use the mjml extensions in the code editor to edit them and so on.

The template names could be the file name, and it would be great if it could be done recursively per directory.