Allow usage as attribute directive

[Maximaximum]

Why was this removed? I've been using the directive all throughout my application, and would love to continue using it that way.

Besides, I'm using the package via npm, and unfortunately the v2.0.0 version (unlike v2.0.0 in bower perhaps) already contains the 'remove directive support' commit.

[isherwood]

See #69. If you'd like to build a PR that reintroduces it and updates the docs, feel free. I figured it was causing more problems than it solved as it was.