Add custom attributes on network buttons

[lucasflomuller]

More details

I've added the possibility to use an options object on each network, the object has three attributes:

{
    icon: {}      // properties will override on the image tag
    text: {}      // properties will override the span tag
    container: {} // properties will override the container tag
}

This closes #13.

I added to the readme how to use it and updated the live demo. I didn't deploy any of it.

[lucasflomuller]

Hi lucas, this looks really good. but instead of nesting the icons into networks, can we do it with a new field icons under the property level? nesting into networks needs a lot other coupling services to update.

Hi, @hao-sharethis, I did what you asked for, hopefully this is what you were requesting 😄

Let me know if you need any further changes, I'll be here!