Ability to override only attributes of data-block.

[sqrter]

Hi Luke!

Thanks for great directive, it's really cool and easy to use. But there is one useful thing that I lack. It's overriding/extending of attributes. Please look at my changes, hope my idea is clear, this feature will make easier to override blocks that have only differences in attributes.

Sergey

[coveralls]

Coverage decreased (-1.0%) when pulling 096b7679bf7e64cefaa958197d7304584f0cc9ad on sqrter:master into a1ee1f7cd3aa43940ef7f7001737ce320220df35 on wmluke:master.