handle "custom data attributes"

[afeld]

http://haml.info/docs/yardoc/file.REFERENCE.html#html5_custom_data_attributes

.text{ :data => { :elem => "text" } }

should be converted to

.text data-elem="text"

Currently those attributes are copied over with the hash directly, which breaks the template. Thanks!

[iRonin]

+1

[minad]

this will work in Slim 2.0 directly