Configuration system implementation

[adrienbanse]

When initializing ecologits with

Ecologits.init(
    providers = provider_list, 
    electricity_mix_zone = "BEL"
)

the static object Config (in ecologits.config) takes the corresponding attributes, and is module-wide accessible.

Remains to

• Implement a verbose argument Implement proper logging
• Write tests
• Write corresponding doc

[adrienbanse]

@samuelrince I didn't implement a verbose argument but a proper logging
Not finding a model or a mix are at the WARNING level
The user can then decide the level himself or herself.

[samuelrince]

I let you @adrienbanse double-check the changes in the documentation, if you are ok with it, you can rebase and merge! :)