Join option to add filename as prefix to header

[tetedange13]

Hi @shenwei356 ,

I tried to implement myself the feature I requested (#202) => Please be aware that these are my first ever written lines of Go

As mentionned :

• First commit does not add prefix in 'firstFile'
• 2nd commit does (including columns used to join on)
  

Hope this helps ! Have nice day, Felix.