SUGGESTION: Add MLS

## Description
I can't get my dad gum amurrican soccer scores

### Versions

- NodeJS version: 8.9.1
- Football CLI version: any

### Error
When I try `football fixtures -l MLS`, nothing happens
 
### Expected behaviour
I should be able to see MLS scores.

## Steps to recreate
Should be self-explanatory.

## People to notify
I hope we can add this to the choices available 😄 
