Add manifest plugin and processor for Dotenv files

[sveetch]

Each app could contain a .env to describe available environment variable that can override the app settings.

The manifest plugin will probably have the same options than the requirements ones.

A new commandline (dotenv) will be needed to compose the final .env file for the project. However it may be created to another filename (.env.composed) to avoid overwriting the .env file that user created to define their effective env vars.