Add default mapping & move functions to autoload/

Hi,

I think goyo can benefit from declaring a default mapping and <Plug> function, that way it would be easier to integrate with vim-plug or similar utilities.

    Plug 'junegunn/goyo.vim', { 'on': '<Plug>Goyo' }

In cases where lazy loading hasn't been setup, I think it could improve the performance moving function definitions to autoload/.