support binding autofocus

[lanterndev]

On 2011-12-20, @IgorMinar said:

with regards to autofocus, the html5 spec says that the focus should be placed on the element when it is "inserted into the document" and there is also an additional restriction that says that there can be no more than one element with attribute autofocus in the document.

so autofocus is quite tricky and requires special treatment. can you please create a new issue for this and reference this issue or just quote this comment.

If multiple elements have a truthy ng-autofocus value, maybe just add the autofocus attribute to the first such visible element?

[pkozlowski-opensource]

Duplicate of #2012