Enhancement for issue #1

Technical reminder about the DB upgrade :
There doesn't seem to be an easy and reusable way to give CMail both
parts of an text+html email, so we rework the database schema to have
a simple 'body' field with a 'body_ishtml' switch, following CMail
interface.

Enhancement from issue #1

Tested with Dolibarr 10.0.7 and 13.0.0.

Since Dolibarr 10.0 just came out, this module may (at last) become
useful. Let's use a more easily understandable name for it.

Tout frais sorti de ModuleBuilder (9.0.0-beta 0fb95c91a4faf0930a6a744dec307d3ffc517157)