Changing data-icon in favor of data-label-icon

data-icon is used by many css fonts like font awesome or css themes like flat-ui and others, using data-icon adds unexpected content because of the css rule ```[data-icon]:before {content: attr(data-icon);}```
Removed the icon-prefix
Merge pull request #104 from Pasvaz/patch-1

Changing data-icon in favor of data-label-icon