add feature of customizing the attribute of avatar img

[Hsinfu]

Here is my way to customize the attribute of avatar img.

1. Take the advantage of kwargs
2. Pass kwargs as parameter to template instead of unrolling it
3. For loop in the template to unroll the kwargs

I also modified the document.

@grantmcconnaughey Can you help to see if there is something missing. Thx.

[coveralls]

Coverage remained the same at 83.4% when pulling 9ab18a7 on Hsinfu:custom_avatar_img_attr into f0c45af on grantmcconnaughey:master.

[grantmcconnaughey]

Nice, I like it. Thanks.