Adding support for a white list of attributes that can be lower case.

When enabling the rule instead of using a boolean, use an array with the attributes (camelCased) that you want to ignore
```json
{
  "attr-lowercase": ["viewBox"]
}
```
Fixes #28

src/rules/attr-lowercase.js

@@ -5,19 +5,20 @@
 HTMLHint.addRule({
     id: 'attr-lowercase',
     description: 'All attribute names must be in lowercase.',
-    init: function(parser, reporter){
+    init: function(parser, reporter, options){
         var self = this;
+        var exceptions = Array.isArray(options) ? options : [];
         parser.addListener('tagstart', function(event){
             var attrs = event.attrs,
                 attr,
                 col = event.col + event.tagName.length + 1;
             for(var i=0, l=attrs.length;i<l;i++){
                 attr = attrs[i];
                 var attrName = attr.name;
-                if(attrName !== attrName.toLowerCase()){
+                if (exceptions.indexOf(attrName) === -1 && attrName !== attrName.toLowerCase()){
                     reporter.error('The attribute name of [ '+attrName+' ] must be in lowercase.', event.line, col + attr.index, self, attr.raw);
                 }
             }
         });
     }
-});
+});