Add a configuration data provider to fetch variables from environment

[jubianchi]

This PR will help automating the Flarum install process. Actually, to bootstrap the initial configuration file the user has to fire a CLI command (php flarum install [--defaults]) which either asks the user to type values or fetch them from a default provider.

With this patch, we have a new provider: DataFromEnvironment. It will read the configuration from environment variables:

export FLARUM_DATABASE_HOST=mysql.local
export FLARUM_DATABASE_NAME=flarum
export FLARUM_DATABASE_USER=flarum
export FLARUM_DATABASE_PASSWORD=flarum
export FLARUM_ADMIN_USERNAME=admin
export FLARUM_ADMIN_PASSWORD=admin
export FLARUM_ADMIN_EMAIL=admin@docker-fl
export FLARUM_FORUM_TITLE=Docker Flarum
export FLARUM_BASE_URL=http://flarum.local

php flarum install --env

This is really useful to provide an automated install for docker for example.

[jubianchi]

I'm thinking of a better solution right now, let me implement it ;)

[jubianchi]

This was a bad idea. sorry for the noise!

[franzliedke]

That's actually a very interesting idea. :)

[jubianchi]

@franzliedke in fact it is but the way I did it was not clean enough to me. I'm experimenting it with an other method and it seems to be ok and does not require any modification in the core ;)

[Mecvak]

This would be extremely useful for installing by script. Hopefully this new idea still utilizes environmental variables!